OK, let's work through this together. We're starting with 891 and we're going to multiply it by 21, which means we add 891 to itself 21 times.
Step 1: 0 + 891 = 891
Step 2: 891 + 891 = 1782
Step 3: 1782 + 891 = 2673
Step 4: 2673 + 891 = 3564
Step 5: 3564 + 891 = 4455
Step 6: 4455 + 891 = 5346
Step 7: 5346 + 891 = 6237
Step 8: 6237 + 891 = 7128
Step 9: 7128 + 891 = 8019
Step 10: 8019 + 891 = 8910
Step 11: 8910 + 891 = 9801
Step 12: 9801 + 891 = 10692
Step 13: 10692 + 891 = 11583
Step 14: 11583 + 891 = 12474
Step 15: 12474 + 891 = 13365
Step 16: 13365 + 891 = 14256
Step 17: 14256 + 891 = 15147
Step 18: 15147 + 891 = 16038
Step 19: 16038 + 891 = 16929
Step 20: 16929 + 891 = 17820
Step 21: 17820 + 891 = 18711

So, 891*21 = 18711