OK, let's crack this. We're given 643 and our task is to multiply it by 6. Essentially, we'll be adding 643 to itself 6 times.
Step 1: 0 + 643 = 643
Step 2: 643 + 643 = 1286
Step 3: 1286 + 643 = 1929
Step 4: 1929 + 643 = 2572
Step 5: 2572 + 643 = 3215
Step 6: 3215 + 643 = 3858

So, 643*6 = 3858