OK, let's work through this together. We're starting with 852 and we're going to multiply it by 21, which means we add 852 to itself 21 times.
Step 1: 0 + 852 = 852
Step 2: 852 + 852 = 1704
Step 3: 1704 + 852 = 2556
Step 4: 2556 + 852 = 3408
Step 5: 3408 + 852 = 4260
Step 6: 4260 + 852 = 5112
Step 7: 5112 + 852 = 5964
Step 8: 5964 + 852 = 6816
Step 9: 6816 + 852 = 7668
Step 10: 7668 + 852 = 8520
Step 11: 8520 + 852 = 9372
Step 12: 9372 + 852 = 10224
Step 13: 10224 + 852 = 11076
Step 14: 11076 + 852 = 11928
Step 15: 11928 + 852 = 12780
Step 16: 12780 + 852 = 13632
Step 17: 13632 + 852 = 14484
Step 18: 14484 + 852 = 15336
Step 19: 15336 + 852 = 16188
Step 20: 16188 + 852 = 17040
Step 21: 17040 + 852 = 17892

So, 852*21 = 17892