Let's roll up our sleeves and solve this. We have 649 and we're going to multiply it by 21, essentially adding 649 to itself 21 times.
Step 1: 0 + 649 = 649
Step 2: 649 + 649 = 1298
Step 3: 1298 + 649 = 1947
Step 4: 1947 + 649 = 2596
Step 5: 2596 + 649 = 3245
Step 6: 3245 + 649 = 3894
Step 7: 3894 + 649 = 4543
Step 8: 4543 + 649 = 5192
Step 9: 5192 + 649 = 5841
Step 10: 5841 + 649 = 6490
Step 11: 6490 + 649 = 7139
Step 12: 7139 + 649 = 7788
Step 13: 7788 + 649 = 8437
Step 14: 8437 + 649 = 9086
Step 15: 9086 + 649 = 9735
Step 16: 9735 + 649 = 10384
Step 17: 10384 + 649 = 11033
Step 18: 11033 + 649 = 11682
Step 19: 11682 + 649 = 12331
Step 20: 12331 + 649 = 12980
Step 21: 12980 + 649 = 13629

So, 649*21 = 13629