OK, let's crack this. We're given 811 and our task is to multiply it by 21. Essentially, we'll be adding 811 to itself 21 times.
Step 1: 0 + 811 = 811
Step 2: 811 + 811 = 1622
Step 3: 1622 + 811 = 2433
Step 4: 2433 + 811 = 3244
Step 5: 3244 + 811 = 4055
Step 6: 4055 + 811 = 4866
Step 7: 4866 + 811 = 5677
Step 8: 5677 + 811 = 6488
Step 9: 6488 + 811 = 7299
Step 10: 7299 + 811 = 8110
Step 11: 8110 + 811 = 8921
Step 12: 8921 + 811 = 9732
Step 13: 9732 + 811 = 10543
Step 14: 10543 + 811 = 11354
Step 15: 11354 + 811 = 12165
Step 16: 12165 + 811 = 12976
Step 17: 12976 + 811 = 13787
Step 18: 13787 + 811 = 14598
Step 19: 14598 + 811 = 15409
Step 20: 15409 + 811 = 16220
Step 21: 16220 + 811 = 17031

So, 811*21 = 17031