OK, let's crack this. We're given 784 and our task is to multiply it by 51. Essentially, we'll be adding 784 to itself 51 times.
Step 1: 0 + 784 = 784
Step 2: 784 + 784 = 1568
Step 3: 1568 + 784 = 2352
Step 4: 2352 + 784 = 3136
Step 5: 3136 + 784 = 3920
Step 6: 3920 + 784 = 4704
Step 7: 4704 + 784 = 5488
Step 8: 5488 + 784 = 6272
Step 9: 6272 + 784 = 7056
Step 10: 7056 + 784 = 7840
Step 11: 7840 + 784 = 8624
Step 12: 8624 + 784 = 9408
Step 13: 9408 + 784 = 10192
Step 14: 10192 + 784 = 10976
Step 15: 10976 + 784 = 11760
Step 16: 11760 + 784 = 12544
Step 17: 12544 + 784 = 13328
Step 18: 13328 + 784 = 14112
Step 19: 14112 + 784 = 14896
Step 20: 14896 + 784 = 15680
Step 21: 15680 + 784 = 16464
Step 22: 16464 + 784 = 17248
Step 23: 17248 + 784 = 18032
Step 24: 18032 + 784 = 18816
Step 25: 18816 + 784 = 19600
Step 26: 19600 + 784 = 20384
Step 27: 20384 + 784 = 21168
Step 28: 21168 + 784 = 21952
Step 29: 21952 + 784 = 22736
Step 30: 22736 + 784 = 23520
Step 31: 23520 + 784 = 24304
Step 32: 24304 + 784 = 25088
Step 33: 25088 + 784 = 25872
Step 34: 25872 + 784 = 26656
Step 35: 26656 + 784 = 27440
Step 36: 27440 + 784 = 28224
Step 37: 28224 + 784 = 29008
Step 38: 29008 + 784 = 29792
Step 39: 29792 + 784 = 30576
Step 40: 30576 + 784 = 31360
Step 41: 31360 + 784 = 32144
Step 42: 32144 + 784 = 32928
Step 43: 32928 + 784 = 33712
Step 44: 33712 + 784 = 34496
Step 45: 34496 + 784 = 35280
Step 46: 35280 + 784 = 36064
Step 47: 36064 + 784 = 36848
Step 48: 36848 + 784 = 37632
Step 49: 37632 + 784 = 38416
Step 50: 38416 + 784 = 39200
Step 51: 39200 + 784 = 39984

So, 784*51 = 39984