Without delay, let's solve this. We've got 639 and we will be multiplying it by 7, that is, adding 639 to itself 7 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

So, 639*7 = 4473