Let's roll up our sleeves and solve this. We have 123 and we're going to multiply it by 143, essentially adding 123 to itself 143 times.
Step 1: 0 + 123 = 123
Step 2: 123 + 123 = 246
Step 3: 246 + 123 = 369
Step 4: 369 + 123 = 492
Step 5: 492 + 123 = 615
Step 6: 615 + 123 = 738
Step 7: 738 + 123 = 861
Step 8: 861 + 123 = 984
Step 9: 984 + 123 = 1107
Step 10: 1107 + 123 = 1230
Step 11: 1230 + 123 = 1353
Step 12: 1353 + 123 = 1476
Step 13: 1476 + 123 = 1599
Step 14: 1599 + 123 = 1722
Step 15: 1722 + 123 = 1845
Step 16: 1845 + 123 = 1968
Step 17: 1968 + 123 = 2091
Step 18: 2091 + 123 = 2214
Step 19: 2214 + 123 = 2337
Step 20: 2337 + 123 = 2460
Step 21: 2460 + 123 = 2583
Step 22: 2583 + 123 = 2706
Step 23: 2706 + 123 = 2829
Step 24: 2829 + 123 = 2952
Step 25: 2952 + 123 = 3075
Step 26: 3075 + 123 = 3198
Step 27: 3198 + 123 = 3321
Step 28: 3321 + 123 = 3444
Step 29: 3444 + 123 = 3567
Step 30: 3567 + 123 = 3690
Step 31: 3690 + 123 = 3813
Step 32: 3813 + 123 = 3936
Step 33: 3936 + 123 = 4059
Step 34: 4059 + 123 = 4182
Step 35: 4182 + 123 = 4305
Step 36: 4305 + 123 = 4428
Step 37: 4428 + 123 = 4551
Step 38: 4551 + 123 = 4674
Step 39: 4674 + 123 = 4797
Step 40: 4797 + 123 = 4920
Step 41: 4920 + 123 = 5043
Step 42: 5043 + 123 = 5166
Step 43: 5166 + 123 = 5289
Step 44: 5289 + 123 = 5412
Step 45: 5412 + 123 = 5535
Step 46: 5535 + 123 = 5658
Step 47: 5658 + 123 = 5781
Step 48: 5781 + 123 = 5904
Step 49: 5904 + 123 = 6027
Step 50: 6027 + 123 = 6150
Step 51: 6150 + 123 = 6273
Step 52: 6273 + 123 = 6396
Step 53: 6396 + 123 = 6519
Step 54: 6519 + 123 = 6642
Step 55: 6642 + 123 = 6765
Step 56: 6765 + 123 = 6888
Step 57: 6888 + 123 = 7011
Step 58: 7011 + 123 = 7134
Step 59: 7134 + 123 = 7257
Step 60: 7257 + 123 = 7380
Step 61: 7380 + 123 = 7503
Step 62: 7503 + 123 = 7626
Step 63: 7626 + 123 = 7749
Step 64: 7749 + 123 = 7872
Step 65: 7872 + 123 = 7995
Step 66: 7995 + 123 = 8118
Step 67: 8118 + 123 = 8241
Step 68: 8241 + 123 = 8364
Step 69: 8364 + 123 = 8487
Step 70: 8487 + 123 = 8610
Step 71: 8610 + 123 = 8733
Step 72: 8733 + 123 = 8856
Step 73: 8856 + 123 = 8979
Step 74: 8979 + 123 = 9102
Step 75: 9102 + 123 = 9225
Step 76: 9225 + 123 = 9348
Step 77: 9348 + 123 = 9471
Step 78: 9471 + 123 = 9594
Step 79: 9594 + 123 = 9717
Step 80: 9717 + 123 = 9840
Step 81: 9840 + 123 = 9963
Step 82: 9963 + 123 = 10086
Step 83: 10086 + 123 = 10209
Step 84: 10209 + 123 = 10332
Step 85: 10332 + 123 = 10455
Step 86: 10455 + 123 = 10578
Step 87: 10578 + 123 = 10701
Step 88: 10701 + 123 = 10824
Step 89: 10824 + 123 = 10947
Step 90: 10947 + 123 = 11070
Step 91: 11070 + 123 = 11193
Step 92: 11193 + 123 = 11316
Step 93: 11316 + 123 = 11439
Step 94: 11439 + 123 = 11562
Step 95: 11562 + 123 = 11685
Step 96: 11685 + 123 = 11808
Step 97: 11808 + 123 = 11931
Step 98: 11931 + 123 = 12054
Step 99: 12054 + 123 = 12177
Step 100: 12177 + 123 = 12300
Step 101: 12300 + 123 = 12423
Step 102: 12423 + 123 = 12546
Step 103: 12546 + 123 = 12669
Step 104: 12669 + 123 = 12792
Step 105: 12792 + 123 = 12915
Step 106: 12915 + 123 = 13038
Step 107: 13038 + 123 = 13161
Step 108: 13161 + 123 = 13284
Step 109: 13284 + 123 = 13407
Step 110: 13407 + 123 = 13530
Step 111: 13530 + 123 = 13653
Step 112: 13653 + 123 = 13776
Step 113: 13776 + 123 = 13899
Step 114: 13899 + 123 = 14022
Step 115: 14022 + 123 = 14145
Step 116: 14145 + 123 = 14268
Step 117: 14268 + 123 = 14391
Step 118: 14391 + 123 = 14514
Step 119: 14514 + 123 = 14637
Step 120: 14637 + 123 = 14760
Step 121: 14760 + 123 = 14883
Step 122: 14883 + 123 = 15006
Step 123: 15006 + 123 = 15129
Step 124: 15129 + 123 = 15252
Step 125: 15252 + 123 = 15375
Step 126: 15375 + 123 = 15498
Step 127: 15498 + 123 = 15621
Step 128: 15621 + 123 = 15744
Step 129: 15744 + 123 = 15867
Step 130: 15867 + 123 = 15990
Step 131: 15990 + 123 = 16113
Step 132: 16113 + 123 = 16236
Step 133: 16236 + 123 = 16359
Step 134: 16359 + 123 = 16482
Step 135: 16482 + 123 = 16605
Step 136: 16605 + 123 = 16728
Step 137: 16728 + 123 = 16851
Step 138: 16851 + 123 = 16974
Step 139: 16974 + 123 = 17097
Step 140: 17097 + 123 = 17220
Step 141: 17220 + 123 = 17343
Step 142: 17343 + 123 = 17466
Step 143: 17466 + 123 = 17589

So, 123*143 = 17589