OK, let's crack this. We're given 931 and our task is to multiply it by 21. Essentially, we'll be adding 931 to itself 21 times.
Step 1: 0 + 931 = 931
Step 2: 931 + 931 = 1862
Step 3: 1862 + 931 = 2793
Step 4: 2793 + 931 = 3724
Step 5: 3724 + 931 = 4655
Step 6: 4655 + 931 = 5586
Step 7: 5586 + 931 = 6517
Step 8: 6517 + 931 = 7448
Step 9: 7448 + 931 = 8379
Step 10: 8379 + 931 = 9310
Step 11: 9310 + 931 = 10241
Step 12: 10241 + 931 = 11172
Step 13: 11172 + 931 = 12103
Step 14: 12103 + 931 = 13034
Step 15: 13034 + 931 = 13965
Step 16: 13965 + 931 = 14896
Step 17: 14896 + 931 = 15827
Step 18: 15827 + 931 = 16758
Step 19: 16758 + 931 = 17689
Step 20: 17689 + 931 = 18620
Step 21: 18620 + 931 = 19551

So, 931*21 = 19551