OK, let's work through this together. We're starting with 981 and we're going to multiply it by 21, which means we add 981 to itself 21 times.
Step 1: 0 + 981 = 981
Step 2: 981 + 981 = 1962
Step 3: 1962 + 981 = 2943
Step 4: 2943 + 981 = 3924
Step 5: 3924 + 981 = 4905
Step 6: 4905 + 981 = 5886
Step 7: 5886 + 981 = 6867
Step 8: 6867 + 981 = 7848
Step 9: 7848 + 981 = 8829
Step 10: 8829 + 981 = 9810
Step 11: 9810 + 981 = 10791
Step 12: 10791 + 981 = 11772
Step 13: 11772 + 981 = 12753
Step 14: 12753 + 981 = 13734
Step 15: 13734 + 981 = 14715
Step 16: 14715 + 981 = 15696
Step 17: 15696 + 981 = 16677
Step 18: 16677 + 981 = 17658
Step 19: 17658 + 981 = 18639
Step 20: 18639 + 981 = 19620
Step 21: 19620 + 981 = 20601

So, 981*21 = 20601