Let's roll up our sleeves and solve this. We have 644 and we're going to multiply it by 68, essentially adding 644 to itself 68 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

So, 644*68 = 43792