Alright, let's solve this math problem step by step. First, we have the number 721. Next, we see the multiplication sign, which means we need to multiply something. And what are we multiplying it by? We multiply it by 21. So, we're going to take the number 721 and add it to itself 21 times.
Step 1: 0 + 721 = 721
Step 2: 721 + 721 = 1442
Step 3: 1442 + 721 = 2163
Step 4: 2163 + 721 = 2884
Step 5: 2884 + 721 = 3605
Step 6: 3605 + 721 = 4326
Step 7: 4326 + 721 = 5047
Step 8: 5047 + 721 = 5768
Step 9: 5768 + 721 = 6489
Step 10: 6489 + 721 = 7210
Step 11: 7210 + 721 = 7931
Step 12: 7931 + 721 = 8652
Step 13: 8652 + 721 = 9373
Step 14: 9373 + 721 = 10094
Step 15: 10094 + 721 = 10815
Step 16: 10815 + 721 = 11536
Step 17: 11536 + 721 = 12257
Step 18: 12257 + 721 = 12978
Step 19: 12978 + 721 = 13699
Step 20: 13699 + 721 = 14420
Step 21: 14420 + 721 = 15141

So, 721*21 = 15141