Sure thing, let's get straight to it. We start with 791 and we're going to multiply it by 21, which means adding 791 to itself 21 times.
Step 1: 0 + 791 = 791
Step 2: 791 + 791 = 1582
Step 3: 1582 + 791 = 2373
Step 4: 2373 + 791 = 3164
Step 5: 3164 + 791 = 3955
Step 6: 3955 + 791 = 4746
Step 7: 4746 + 791 = 5537
Step 8: 5537 + 791 = 6328
Step 9: 6328 + 791 = 7119
Step 10: 7119 + 791 = 7910
Step 11: 7910 + 791 = 8701
Step 12: 8701 + 791 = 9492
Step 13: 9492 + 791 = 10283
Step 14: 10283 + 791 = 11074
Step 15: 11074 + 791 = 11865
Step 16: 11865 + 791 = 12656
Step 17: 12656 + 791 = 13447
Step 18: 13447 + 791 = 14238
Step 19: 14238 + 791 = 15029
Step 20: 15029 + 791 = 15820
Step 21: 15820 + 791 = 16611

So, 791*21 = 16611