Without delay, let's solve this. We've got 972 and we will be multiplying it by 21, that is, adding 972 to itself 21 times.
Step 1: 0 + 972 = 972
Step 2: 972 + 972 = 1944
Step 3: 1944 + 972 = 2916
Step 4: 2916 + 972 = 3888
Step 5: 3888 + 972 = 4860
Step 6: 4860 + 972 = 5832
Step 7: 5832 + 972 = 6804
Step 8: 6804 + 972 = 7776
Step 9: 7776 + 972 = 8748
Step 10: 8748 + 972 = 9720
Step 11: 9720 + 972 = 10692
Step 12: 10692 + 972 = 11664
Step 13: 11664 + 972 = 12636
Step 14: 12636 + 972 = 13608
Step 15: 13608 + 972 = 14580
Step 16: 14580 + 972 = 15552
Step 17: 15552 + 972 = 16524
Step 18: 16524 + 972 = 17496
Step 19: 17496 + 972 = 18468
Step 20: 18468 + 972 = 19440
Step 21: 19440 + 972 = 20412

So, 972*21 = 20412