OK, let's crack this. We're given 639 and our task is to multiply it by 21. Essentially, we'll be adding 639 to itself 21 times.
Step 1: 0 + 639 = 639
Step 2: 639 + 639 = 1278
Step 3: 1278 + 639 = 1917
Step 4: 1917 + 639 = 2556
Step 5: 2556 + 639 = 3195
Step 6: 3195 + 639 = 3834
Step 7: 3834 + 639 = 4473
Step 8: 4473 + 639 = 5112
Step 9: 5112 + 639 = 5751
Step 10: 5751 + 639 = 6390
Step 11: 6390 + 639 = 7029
Step 12: 7029 + 639 = 7668
Step 13: 7668 + 639 = 8307
Step 14: 8307 + 639 = 8946
Step 15: 8946 + 639 = 9585
Step 16: 9585 + 639 = 10224
Step 17: 10224 + 639 = 10863
Step 18: 10863 + 639 = 11502
Step 19: 11502 + 639 = 12141
Step 20: 12141 + 639 = 12780
Step 21: 12780 + 639 = 13419

So, 639*21 = 13419