Let's roll up our sleeves and solve this. We have 881 and we're going to multiply it by 21, essentially adding 881 to itself 21 times.
Step 1: 0 + 881 = 881
Step 2: 881 + 881 = 1762
Step 3: 1762 + 881 = 2643
Step 4: 2643 + 881 = 3524
Step 5: 3524 + 881 = 4405
Step 6: 4405 + 881 = 5286
Step 7: 5286 + 881 = 6167
Step 8: 6167 + 881 = 7048
Step 9: 7048 + 881 = 7929
Step 10: 7929 + 881 = 8810
Step 11: 8810 + 881 = 9691
Step 12: 9691 + 881 = 10572
Step 13: 10572 + 881 = 11453
Step 14: 11453 + 881 = 12334
Step 15: 12334 + 881 = 13215
Step 16: 13215 + 881 = 14096
Step 17: 14096 + 881 = 14977
Step 18: 14977 + 881 = 15858
Step 19: 15858 + 881 = 16739
Step 20: 16739 + 881 = 17620
Step 21: 17620 + 881 = 18501

So, 881*21 = 18501