Let's get this math done. We have 993 and we're going to multiply it by 21. This is the same as taking 993 and adding it to itself 21 times.
Step 1: 0 + 993 = 993
Step 2: 993 + 993 = 1986
Step 3: 1986 + 993 = 2979
Step 4: 2979 + 993 = 3972
Step 5: 3972 + 993 = 4965
Step 6: 4965 + 993 = 5958
Step 7: 5958 + 993 = 6951
Step 8: 6951 + 993 = 7944
Step 9: 7944 + 993 = 8937
Step 10: 8937 + 993 = 9930
Step 11: 9930 + 993 = 10923
Step 12: 10923 + 993 = 11916
Step 13: 11916 + 993 = 12909
Step 14: 12909 + 993 = 13902
Step 15: 13902 + 993 = 14895
Step 16: 14895 + 993 = 15888
Step 17: 15888 + 993 = 16881
Step 18: 16881 + 993 = 17874
Step 19: 17874 + 993 = 18867
Step 20: 18867 + 993 = 19860
Step 21: 19860 + 993 = 20853

So, 993*21 = 20853