Let's roll up our sleeves and solve this. We have 639 and we're going to multiply it by 12, essentially adding 639 to itself 12 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

So, 639*12 = 7668