OK, let's crack this. We're given 21 and our task is to multiply it by 6. Essentially, we'll be adding 21 to itself 6 times.
Step 1: 0 + 21 = 21
Step 2: 21 + 21 = 42
Step 3: 42 + 21 = 63
Step 4: 63 + 21 = 84
Step 5: 84 + 21 = 105
Step 6: 105 + 21 = 126

So, 21*6 = 126