Let's roll up our sleeves and solve this. We have 633 and we're going to multiply it by 21, essentially adding 633 to itself 21 times.
Step 1: 0 + 633 = 633
Step 2: 633 + 633 = 1266
Step 3: 1266 + 633 = 1899
Step 4: 1899 + 633 = 2532
Step 5: 2532 + 633 = 3165
Step 6: 3165 + 633 = 3798
Step 7: 3798 + 633 = 4431
Step 8: 4431 + 633 = 5064
Step 9: 5064 + 633 = 5697
Step 10: 5697 + 633 = 6330
Step 11: 6330 + 633 = 6963
Step 12: 6963 + 633 = 7596
Step 13: 7596 + 633 = 8229
Step 14: 8229 + 633 = 8862
Step 15: 8862 + 633 = 9495
Step 16: 9495 + 633 = 10128
Step 17: 10128 + 633 = 10761
Step 18: 10761 + 633 = 11394
Step 19: 11394 + 633 = 12027
Step 20: 12027 + 633 = 12660
Step 21: 12660 + 633 = 13293

So, 633*21 = 13293