Alright, let's solve this math problem step by step. First, we have the number 621. 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 621 and add it to itself 21 times.
Step 1: 0 + 621 = 621
Step 2: 621 + 621 = 1242
Step 3: 1242 + 621 = 1863
Step 4: 1863 + 621 = 2484
Step 5: 2484 + 621 = 3105
Step 6: 3105 + 621 = 3726
Step 7: 3726 + 621 = 4347
Step 8: 4347 + 621 = 4968
Step 9: 4968 + 621 = 5589
Step 10: 5589 + 621 = 6210
Step 11: 6210 + 621 = 6831
Step 12: 6831 + 621 = 7452
Step 13: 7452 + 621 = 8073
Step 14: 8073 + 621 = 8694
Step 15: 8694 + 621 = 9315
Step 16: 9315 + 621 = 9936
Step 17: 9936 + 621 = 10557
Step 18: 10557 + 621 = 11178
Step 19: 11178 + 621 = 11799
Step 20: 11799 + 621 = 12420
Step 21: 12420 + 621 = 13041

So, 621*21 = 13041