OK, let's work through this together. We're starting with 721 and we're going to multiply it by 399, which means we add 721 to itself 399 times.
Step 1: 0 + 721 = 721
Step 2: 721 + 721 = 1442
Step 3: 1442 + 721 = 2163
Step 4: 2163 + 721 = 2884
Step 5: 2884 + 721 = 3605
Step 6: 3605 + 721 = 4326
Step 7: 4326 + 721 = 5047
Step 8: 5047 + 721 = 5768
Step 9: 5768 + 721 = 6489
Step 10: 6489 + 721 = 7210
Step 11: 7210 + 721 = 7931
Step 12: 7931 + 721 = 8652
Step 13: 8652 + 721 = 9373
Step 14: 9373 + 721 = 10094
Step 15: 10094 + 721 = 10815
Step 16: 10815 + 721 = 11536
Step 17: 11536 + 721 = 12257
Step 18: 12257 + 721 = 12978
Step 19: 12978 + 721 = 13699
Step 20: 13699 + 721 = 14420
Step 21: 14420 + 721 = 15141
Step 22: 15141 + 721 = 15862
Step 23: 15862 + 721 = 16583
Step 24: 16583 + 721 = 17304
Step 25: 17304 + 721 = 18025
Step 26: 18025 + 721 = 18746
Step 27: 18746 + 721 = 19467
Step 28: 19467 + 721 = 20188
Step 29: 20188 + 721 = 20909
Step 30: 20909 + 721 = 21630
Step 31: 21630 + 721 = 22351
Step 32: 22351 + 721 = 23072
Step 33: 23072 + 721 = 23793
Step 34: 23793 + 721 = 24514
Step 35: 24514 + 721 = 25235
Step 36: 25235 + 721 = 25956
Step 37: 25956 + 721 = 26677
Step 38: 26677 + 721 = 27398
Step 39: 27398 + 721 = 28119
Step 40: 28119 + 721 = 28840
Step 41: 28840 + 721 = 29561
Step 42: 29561 + 721 = 30282
Step 43: 30282 + 721 = 31003
Step 44: 31003 + 721 = 31724
Step 45: 31724 + 721 = 32445
Step 46: 32445 + 721 = 33166
Step 47: 33166 + 721 = 33887
Step 48: 33887 + 721 = 34608
Step 49: 34608 + 721 = 35329
Step 50: 35329 + 721 = 36050
Step 51: 36050 + 721 = 36771
Step 52: 36771 + 721 = 37492
Step 53: 37492 + 721 = 38213
Step 54: 38213 + 721 = 38934
Step 55: 38934 + 721 = 39655
Step 56: 39655 + 721 = 40376
Step 57: 40376 + 721 = 41097
Step 58: 41097 + 721 = 41818
Step 59: 41818 + 721 = 42539
Step 60: 42539 + 721 = 43260
Step 61: 43260 + 721 = 43981
Step 62: 43981 + 721 = 44702
Step 63: 44702 + 721 = 45423
Step 64: 45423 + 721 = 46144
Step 65: 46144 + 721 = 46865
Step 66: 46865 + 721 = 47586
Step 67: 47586 + 721 = 48307
Step 68: 48307 + 721 = 49028
Step 69: 49028 + 721 = 49749
Step 70: 49749 + 721 = 50470
Step 71: 50470 + 721 = 51191
Step 72: 51191 + 721 = 51912
Step 73: 51912 + 721 = 52633
Step 74: 52633 + 721 = 53354
Step 75: 53354 + 721 = 54075
Step 76: 54075 + 721 = 54796
Step 77: 54796 + 721 = 55517
Step 78: 55517 + 721 = 56238
Step 79: 56238 + 721 = 56959
Step 80: 56959 + 721 = 57680
Step 81: 57680 + 721 = 58401
Step 82: 58401 + 721 = 59122
Step 83: 59122 + 721 = 59843
Step 84: 59843 + 721 = 60564
Step 85: 60564 + 721 = 61285
Step 86: 61285 + 721 = 62006
Step 87: 62006 + 721 = 62727
Step 88: 62727 + 721 = 63448
Step 89: 63448 + 721 = 64169
Step 90: 64169 + 721 = 64890
Step 91: 64890 + 721 = 65611
Step 92: 65611 + 721 = 66332
Step 93: 66332 + 721 = 67053
Step 94: 67053 + 721 = 67774
Step 95: 67774 + 721 = 68495
Step 96: 68495 + 721 = 69216
Step 97: 69216 + 721 = 69937
Step 98: 69937 + 721 = 70658
Step 99: 70658 + 721 = 71379
Step 100: 71379 + 721 = 72100
Step 101: 72100 + 721 = 72821
Step 102: 72821 + 721 = 73542
Step 103: 73542 + 721 = 74263
Step 104: 74263 + 721 = 74984
Step 105: 74984 + 721 = 75705
Step 106: 75705 + 721 = 76426
Step 107: 76426 + 721 = 77147
Step 108: 77147 + 721 = 77868
Step 109: 77868 + 721 = 78589
Step 110: 78589 + 721 = 79310
Step 111: 79310 + 721 = 80031
Step 112: 80031 + 721 = 80752
Step 113: 80752 + 721 = 81473
Step 114: 81473 + 721 = 82194
Step 115: 82194 + 721 = 82915
Step 116: 82915 + 721 = 83636
Step 117: 83636 + 721 = 84357
Step 118: 84357 + 721 = 85078
Step 119: 85078 + 721 = 85799
Step 120: 85799 + 721 = 86520
Step 121: 86520 + 721 = 87241
Step 122: 87241 + 721 = 87962
Step 123: 87962 + 721 = 88683
Step 124: 88683 + 721 = 89404
Step 125: 89404 + 721 = 90125
Step 126: 90125 + 721 = 90846
Step 127: 90846 + 721 = 91567
Step 128: 91567 + 721 = 92288
Step 129: 92288 + 721 = 93009
Step 130: 93009 + 721 = 93730
Step 131: 93730 + 721 = 94451
Step 132: 94451 + 721 = 95172
Step 133: 95172 + 721 = 95893
Step 134: 95893 + 721 = 96614
Step 135: 96614 + 721 = 97335
Step 136: 97335 + 721 = 98056
Step 137: 98056 + 721 = 98777
Step 138: 98777 + 721 = 99498
Step 139: 99498 + 721 = 100219
Step 140: 100219 + 721 = 100940
Step 141: 100940 + 721 = 101661
Step 142: 101661 + 721 = 102382
Step 143: 102382 + 721 = 103103
Step 144: 103103 + 721 = 103824
Step 145: 103824 + 721 = 104545
Step 146: 104545 + 721 = 105266
Step 147: 105266 + 721 = 105987
Step 148: 105987 + 721 = 106708
Step 149: 106708 + 721 = 107429
Step 150: 107429 + 721 = 108150
Step 151: 108150 + 721 = 108871
Step 152: 108871 + 721 = 109592
Step 153: 109592 + 721 = 110313
Step 154: 110313 + 721 = 111034
Step 155: 111034 + 721 = 111755
Step 156: 111755 + 721 = 112476
Step 157: 112476 + 721 = 113197
Step 158: 113197 + 721 = 113918
Step 159: 113918 + 721 = 114639
Step 160: 114639 + 721 = 115360
Step 161: 115360 + 721 = 116081
Step 162: 116081 + 721 = 116802
Step 163: 116802 + 721 = 117523
Step 164: 117523 + 721 = 118244
Step 165: 118244 + 721 = 118965
Step 166: 118965 + 721 = 119686
Step 167: 119686 + 721 = 120407
Step 168: 120407 + 721 = 121128
Step 169: 121128 + 721 = 121849
Step 170: 121849 + 721 = 122570
Step 171: 122570 + 721 = 123291
Step 172: 123291 + 721 = 124012
Step 173: 124012 + 721 = 124733
Step 174: 124733 + 721 = 125454
Step 175: 125454 + 721 = 126175
Step 176: 126175 + 721 = 126896
Step 177: 126896 + 721 = 127617
Step 178: 127617 + 721 = 128338
Step 179: 128338 + 721 = 129059
Step 180: 129059 + 721 = 129780
Step 181: 129780 + 721 = 130501
Step 182: 130501 + 721 = 131222
Step 183: 131222 + 721 = 131943
Step 184: 131943 + 721 = 132664
Step 185: 132664 + 721 = 133385
Step 186: 133385 + 721 = 134106
Step 187: 134106 + 721 = 134827
Step 188: 134827 + 721 = 135548
Step 189: 135548 + 721 = 136269
Step 190: 136269 + 721 = 136990
Step 191: 136990 + 721 = 137711
Step 192: 137711 + 721 = 138432
Step 193: 138432 + 721 = 139153
Step 194: 139153 + 721 = 139874
Step 195: 139874 + 721 = 140595
Step 196: 140595 + 721 = 141316
Step 197: 141316 + 721 = 142037
Step 198: 142037 + 721 = 142758
Step 199: 142758 + 721 = 143479
Step 200: 143479 + 721 = 144200
Step 201: 144200 + 721 = 144921
Step 202: 144921 + 721 = 145642
Step 203: 145642 + 721 = 146363
Step 204: 146363 + 721 = 147084
Step 205: 147084 + 721 = 147805
Step 206: 147805 + 721 = 148526
Step 207: 148526 + 721 = 149247
Step 208: 149247 + 721 = 149968
Step 209: 149968 + 721 = 150689
Step 210: 150689 + 721 = 151410
Step 211: 151410 + 721 = 152131
Step 212: 152131 + 721 = 152852
Step 213: 152852 + 721 = 153573
Step 214: 153573 + 721 = 154294
Step 215: 154294 + 721 = 155015
Step 216: 155015 + 721 = 155736
Step 217: 155736 + 721 = 156457
Step 218: 156457 + 721 = 157178
Step 219: 157178 + 721 = 157899
Step 220: 157899 + 721 = 158620
Step 221: 158620 + 721 = 159341
Step 222: 159341 + 721 = 160062
Step 223: 160062 + 721 = 160783
Step 224: 160783 + 721 = 161504
Step 225: 161504 + 721 = 162225
Step 226: 162225 + 721 = 162946
Step 227: 162946 + 721 = 163667
Step 228: 163667 + 721 = 164388
Step 229: 164388 + 721 = 165109
Step 230: 165109 + 721 = 165830
Step 231: 165830 + 721 = 166551
Step 232: 166551 + 721 = 167272
Step 233: 167272 + 721 = 167993
Step 234: 167993 + 721 = 168714
Step 235: 168714 + 721 = 169435
Step 236: 169435 + 721 = 170156
Step 237: 170156 + 721 = 170877
Step 238: 170877 + 721 = 171598
Step 239: 171598 + 721 = 172319
Step 240: 172319 + 721 = 173040
Step 241: 173040 + 721 = 173761
Step 242: 173761 + 721 = 174482
Step 243: 174482 + 721 = 175203
Step 244: 175203 + 721 = 175924
Step 245: 175924 + 721 = 176645
Step 246: 176645 + 721 = 177366
Step 247: 177366 + 721 = 178087
Step 248: 178087 + 721 = 178808
Step 249: 178808 + 721 = 179529
Step 250: 179529 + 721 = 180250
Step 251: 180250 + 721 = 180971
Step 252: 180971 + 721 = 181692
Step 253: 181692 + 721 = 182413
Step 254: 182413 + 721 = 183134
Step 255: 183134 + 721 = 183855
Step 256: 183855 + 721 = 184576
Step 257: 184576 + 721 = 185297
Step 258: 185297 + 721 = 186018
Step 259: 186018 + 721 = 186739
Step 260: 186739 + 721 = 187460
Step 261: 187460 + 721 = 188181
Step 262: 188181 + 721 = 188902
Step 263: 188902 + 721 = 189623
Step 264: 189623 + 721 = 190344
Step 265: 190344 + 721 = 191065
Step 266: 191065 + 721 = 191786
Step 267: 191786 + 721 = 192507
Step 268: 192507 + 721 = 193228
Step 269: 193228 + 721 = 193949
Step 270: 193949 + 721 = 194670
Step 271: 194670 + 721 = 195391
Step 272: 195391 + 721 = 196112
Step 273: 196112 + 721 = 196833
Step 274: 196833 + 721 = 197554
Step 275: 197554 + 721 = 198275
Step 276: 198275 + 721 = 198996
Step 277: 198996 + 721 = 199717
Step 278: 199717 + 721 = 200438
Step 279: 200438 + 721 = 201159
Step 280: 201159 + 721 = 201880
Step 281: 201880 + 721 = 202601
Step 282: 202601 + 721 = 203322
Step 283: 203322 + 721 = 204043
Step 284: 204043 + 721 = 204764
Step 285: 204764 + 721 = 205485
Step 286: 205485 + 721 = 206206
Step 287: 206206 + 721 = 206927
Step 288: 206927 + 721 = 207648
Step 289: 207648 + 721 = 208369
Step 290: 208369 + 721 = 209090
Step 291: 209090 + 721 = 209811
Step 292: 209811 + 721 = 210532
Step 293: 210532 + 721 = 211253
Step 294: 211253 + 721 = 211974
Step 295: 211974 + 721 = 212695
Step 296: 212695 + 721 = 213416
Step 297: 213416 + 721 = 214137
Step 298: 214137 + 721 = 214858
Step 299: 214858 + 721 = 215579
Step 300: 215579 + 721 = 216300
Step 301: 216300 + 721 = 217021
Step 302: 217021 + 721 = 217742
Step 303: 217742 + 721 = 218463
Step 304: 218463 + 721 = 219184
Step 305: 219184 + 721 = 219905
Step 306: 219905 + 721 = 220626
Step 307: 220626 + 721 = 221347
Step 308: 221347 + 721 = 222068
Step 309: 222068 + 721 = 222789
Step 310: 222789 + 721 = 223510
Step 311: 223510 + 721 = 224231
Step 312: 224231 + 721 = 224952
Step 313: 224952 + 721 = 225673
Step 314: 225673 + 721 = 226394
Step 315: 226394 + 721 = 227115
Step 316: 227115 + 721 = 227836
Step 317: 227836 + 721 = 228557
Step 318: 228557 + 721 = 229278
Step 319: 229278 + 721 = 229999
Step 320: 229999 + 721 = 230720
Step 321: 230720 + 721 = 231441
Step 322: 231441 + 721 = 232162
Step 323: 232162 + 721 = 232883
Step 324: 232883 + 721 = 233604
Step 325: 233604 + 721 = 234325
Step 326: 234325 + 721 = 235046
Step 327: 235046 + 721 = 235767
Step 328: 235767 + 721 = 236488
Step 329: 236488 + 721 = 237209
Step 330: 237209 + 721 = 237930
Step 331: 237930 + 721 = 238651
Step 332: 238651 + 721 = 239372
Step 333: 239372 + 721 = 240093
Step 334: 240093 + 721 = 240814
Step 335: 240814 + 721 = 241535
Step 336: 241535 + 721 = 242256
Step 337: 242256 + 721 = 242977
Step 338: 242977 + 721 = 243698
Step 339: 243698 + 721 = 244419
Step 340: 244419 + 721 = 245140
Step 341: 245140 + 721 = 245861
Step 342: 245861 + 721 = 246582
Step 343: 246582 + 721 = 247303
Step 344: 247303 + 721 = 248024
Step 345: 248024 + 721 = 248745
Step 346: 248745 + 721 = 249466
Step 347: 249466 + 721 = 250187
Step 348: 250187 + 721 = 250908
Step 349: 250908 + 721 = 251629
Step 350: 251629 + 721 = 252350
Step 351: 252350 + 721 = 253071
Step 352: 253071 + 721 = 253792
Step 353: 253792 + 721 = 254513
Step 354: 254513 + 721 = 255234
Step 355: 255234 + 721 = 255955
Step 356: 255955 + 721 = 256676
Step 357: 256676 + 721 = 257397
Step 358: 257397 + 721 = 258118
Step 359: 258118 + 721 = 258839
Step 360: 258839 + 721 = 259560
Step 361: 259560 + 721 = 260281
Step 362: 260281 + 721 = 261002
Step 363: 261002 + 721 = 261723
Step 364: 261723 + 721 = 262444
Step 365: 262444 + 721 = 263165
Step 366: 263165 + 721 = 263886
Step 367: 263886 + 721 = 264607
Step 368: 264607 + 721 = 265328
Step 369: 265328 + 721 = 266049
Step 370: 266049 + 721 = 266770
Step 371: 266770 + 721 = 267491
Step 372: 267491 + 721 = 268212
Step 373: 268212 + 721 = 268933
Step 374: 268933 + 721 = 269654
Step 375: 269654 + 721 = 270375
Step 376: 270375 + 721 = 271096
Step 377: 271096 + 721 = 271817
Step 378: 271817 + 721 = 272538
Step 379: 272538 + 721 = 273259
Step 380: 273259 + 721 = 273980
Step 381: 273980 + 721 = 274701
Step 382: 274701 + 721 = 275422
Step 383: 275422 + 721 = 276143
Step 384: 276143 + 721 = 276864
Step 385: 276864 + 721 = 277585
Step 386: 277585 + 721 = 278306
Step 387: 278306 + 721 = 279027
Step 388: 279027 + 721 = 279748
Step 389: 279748 + 721 = 280469
Step 390: 280469 + 721 = 281190
Step 391: 281190 + 721 = 281911
Step 392: 281911 + 721 = 282632
Step 393: 282632 + 721 = 283353
Step 394: 283353 + 721 = 284074
Step 395: 284074 + 721 = 284795
Step 396: 284795 + 721 = 285516
Step 397: 285516 + 721 = 286237
Step 398: 286237 + 721 = 286958
Step 399: 286958 + 721 = 287679

So, 721*399 = 287679