Without delay, let's solve this. We've got 786 and we will be multiplying it by 21, that is, adding 786 to itself 21 times.
Step 1: 0 + 786 = 786
Step 2: 786 + 786 = 1572
Step 3: 1572 + 786 = 2358
Step 4: 2358 + 786 = 3144
Step 5: 3144 + 786 = 3930
Step 6: 3930 + 786 = 4716
Step 7: 4716 + 786 = 5502
Step 8: 5502 + 786 = 6288
Step 9: 6288 + 786 = 7074
Step 10: 7074 + 786 = 7860
Step 11: 7860 + 786 = 8646
Step 12: 8646 + 786 = 9432
Step 13: 9432 + 786 = 10218
Step 14: 10218 + 786 = 11004
Step 15: 11004 + 786 = 11790
Step 16: 11790 + 786 = 12576
Step 17: 12576 + 786 = 13362
Step 18: 13362 + 786 = 14148
Step 19: 14148 + 786 = 14934
Step 20: 14934 + 786 = 15720
Step 21: 15720 + 786 = 16506

So, 786*21 = 16506