Alright, let's solve this math problem step by step. First, we have the number 657. 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 657 and add it to itself 21 times.
Step 1: 0 + 657 = 657
Step 2: 657 + 657 = 1314
Step 3: 1314 + 657 = 1971
Step 4: 1971 + 657 = 2628
Step 5: 2628 + 657 = 3285
Step 6: 3285 + 657 = 3942
Step 7: 3942 + 657 = 4599
Step 8: 4599 + 657 = 5256
Step 9: 5256 + 657 = 5913
Step 10: 5913 + 657 = 6570
Step 11: 6570 + 657 = 7227
Step 12: 7227 + 657 = 7884
Step 13: 7884 + 657 = 8541
Step 14: 8541 + 657 = 9198
Step 15: 9198 + 657 = 9855
Step 16: 9855 + 657 = 10512
Step 17: 10512 + 657 = 11169
Step 18: 11169 + 657 = 11826
Step 19: 11826 + 657 = 12483
Step 20: 12483 + 657 = 13140
Step 21: 13140 + 657 = 13797

So, 657*21 = 13797