OK, let's crack this. We're given 644 and our task is to multiply it by 173. Essentially, we'll be adding 644 to itself 173 times.
Step 1: 0 + 644 = 644
Step 2: 644 + 644 = 1288
Step 3: 1288 + 644 = 1932
Step 4: 1932 + 644 = 2576
Step 5: 2576 + 644 = 3220
Step 6: 3220 + 644 = 3864
Step 7: 3864 + 644 = 4508
Step 8: 4508 + 644 = 5152
Step 9: 5152 + 644 = 5796
Step 10: 5796 + 644 = 6440
Step 11: 6440 + 644 = 7084
Step 12: 7084 + 644 = 7728
Step 13: 7728 + 644 = 8372
Step 14: 8372 + 644 = 9016
Step 15: 9016 + 644 = 9660
Step 16: 9660 + 644 = 10304
Step 17: 10304 + 644 = 10948
Step 18: 10948 + 644 = 11592
Step 19: 11592 + 644 = 12236
Step 20: 12236 + 644 = 12880
Step 21: 12880 + 644 = 13524
Step 22: 13524 + 644 = 14168
Step 23: 14168 + 644 = 14812
Step 24: 14812 + 644 = 15456
Step 25: 15456 + 644 = 16100
Step 26: 16100 + 644 = 16744
Step 27: 16744 + 644 = 17388
Step 28: 17388 + 644 = 18032
Step 29: 18032 + 644 = 18676
Step 30: 18676 + 644 = 19320
Step 31: 19320 + 644 = 19964
Step 32: 19964 + 644 = 20608
Step 33: 20608 + 644 = 21252
Step 34: 21252 + 644 = 21896
Step 35: 21896 + 644 = 22540
Step 36: 22540 + 644 = 23184
Step 37: 23184 + 644 = 23828
Step 38: 23828 + 644 = 24472
Step 39: 24472 + 644 = 25116
Step 40: 25116 + 644 = 25760
Step 41: 25760 + 644 = 26404
Step 42: 26404 + 644 = 27048
Step 43: 27048 + 644 = 27692
Step 44: 27692 + 644 = 28336
Step 45: 28336 + 644 = 28980
Step 46: 28980 + 644 = 29624
Step 47: 29624 + 644 = 30268
Step 48: 30268 + 644 = 30912
Step 49: 30912 + 644 = 31556
Step 50: 31556 + 644 = 32200
Step 51: 32200 + 644 = 32844
Step 52: 32844 + 644 = 33488
Step 53: 33488 + 644 = 34132
Step 54: 34132 + 644 = 34776
Step 55: 34776 + 644 = 35420
Step 56: 35420 + 644 = 36064
Step 57: 36064 + 644 = 36708
Step 58: 36708 + 644 = 37352
Step 59: 37352 + 644 = 37996
Step 60: 37996 + 644 = 38640
Step 61: 38640 + 644 = 39284
Step 62: 39284 + 644 = 39928
Step 63: 39928 + 644 = 40572
Step 64: 40572 + 644 = 41216
Step 65: 41216 + 644 = 41860
Step 66: 41860 + 644 = 42504
Step 67: 42504 + 644 = 43148
Step 68: 43148 + 644 = 43792
Step 69: 43792 + 644 = 44436
Step 70: 44436 + 644 = 45080
Step 71: 45080 + 644 = 45724
Step 72: 45724 + 644 = 46368
Step 73: 46368 + 644 = 47012
Step 74: 47012 + 644 = 47656
Step 75: 47656 + 644 = 48300
Step 76: 48300 + 644 = 48944
Step 77: 48944 + 644 = 49588
Step 78: 49588 + 644 = 50232
Step 79: 50232 + 644 = 50876
Step 80: 50876 + 644 = 51520
Step 81: 51520 + 644 = 52164
Step 82: 52164 + 644 = 52808
Step 83: 52808 + 644 = 53452
Step 84: 53452 + 644 = 54096
Step 85: 54096 + 644 = 54740
Step 86: 54740 + 644 = 55384
Step 87: 55384 + 644 = 56028
Step 88: 56028 + 644 = 56672
Step 89: 56672 + 644 = 57316
Step 90: 57316 + 644 = 57960
Step 91: 57960 + 644 = 58604
Step 92: 58604 + 644 = 59248
Step 93: 59248 + 644 = 59892
Step 94: 59892 + 644 = 60536
Step 95: 60536 + 644 = 61180
Step 96: 61180 + 644 = 61824
Step 97: 61824 + 644 = 62468
Step 98: 62468 + 644 = 63112
Step 99: 63112 + 644 = 63756
Step 100: 63756 + 644 = 64400
Step 101: 64400 + 644 = 65044
Step 102: 65044 + 644 = 65688
Step 103: 65688 + 644 = 66332
Step 104: 66332 + 644 = 66976
Step 105: 66976 + 644 = 67620
Step 106: 67620 + 644 = 68264
Step 107: 68264 + 644 = 68908
Step 108: 68908 + 644 = 69552
Step 109: 69552 + 644 = 70196
Step 110: 70196 + 644 = 70840
Step 111: 70840 + 644 = 71484
Step 112: 71484 + 644 = 72128
Step 113: 72128 + 644 = 72772
Step 114: 72772 + 644 = 73416
Step 115: 73416 + 644 = 74060
Step 116: 74060 + 644 = 74704
Step 117: 74704 + 644 = 75348
Step 118: 75348 + 644 = 75992
Step 119: 75992 + 644 = 76636
Step 120: 76636 + 644 = 77280
Step 121: 77280 + 644 = 77924
Step 122: 77924 + 644 = 78568
Step 123: 78568 + 644 = 79212
Step 124: 79212 + 644 = 79856
Step 125: 79856 + 644 = 80500
Step 126: 80500 + 644 = 81144
Step 127: 81144 + 644 = 81788
Step 128: 81788 + 644 = 82432
Step 129: 82432 + 644 = 83076
Step 130: 83076 + 644 = 83720
Step 131: 83720 + 644 = 84364
Step 132: 84364 + 644 = 85008
Step 133: 85008 + 644 = 85652
Step 134: 85652 + 644 = 86296
Step 135: 86296 + 644 = 86940
Step 136: 86940 + 644 = 87584
Step 137: 87584 + 644 = 88228
Step 138: 88228 + 644 = 88872
Step 139: 88872 + 644 = 89516
Step 140: 89516 + 644 = 90160
Step 141: 90160 + 644 = 90804
Step 142: 90804 + 644 = 91448
Step 143: 91448 + 644 = 92092
Step 144: 92092 + 644 = 92736
Step 145: 92736 + 644 = 93380
Step 146: 93380 + 644 = 94024
Step 147: 94024 + 644 = 94668
Step 148: 94668 + 644 = 95312
Step 149: 95312 + 644 = 95956
Step 150: 95956 + 644 = 96600
Step 151: 96600 + 644 = 97244
Step 152: 97244 + 644 = 97888
Step 153: 97888 + 644 = 98532
Step 154: 98532 + 644 = 99176
Step 155: 99176 + 644 = 99820
Step 156: 99820 + 644 = 100464
Step 157: 100464 + 644 = 101108
Step 158: 101108 + 644 = 101752
Step 159: 101752 + 644 = 102396
Step 160: 102396 + 644 = 103040
Step 161: 103040 + 644 = 103684
Step 162: 103684 + 644 = 104328
Step 163: 104328 + 644 = 104972
Step 164: 104972 + 644 = 105616
Step 165: 105616 + 644 = 106260
Step 166: 106260 + 644 = 106904
Step 167: 106904 + 644 = 107548
Step 168: 107548 + 644 = 108192
Step 169: 108192 + 644 = 108836
Step 170: 108836 + 644 = 109480
Step 171: 109480 + 644 = 110124
Step 172: 110124 + 644 = 110768
Step 173: 110768 + 644 = 111412

So, 644*173 = 111412