Without delay, let's solve this. We've got 21 and we will be multiplying it by 7, that is, adding 21 to itself 7 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
Step 7: 126 + 21 = 147

So, 21*7 = 147