Alright, let's solve this math problem step by step. First, we have the number 921. Next, we see the multiplication sign, which means we need to multiply something. And what are we multiplying it by? We multiply it by 15. So, we're going to take the number 921 and add it to itself 15 times.
Step 1: 0 + 921 = 921
Step 2: 921 + 921 = 1842
Step 3: 1842 + 921 = 2763
Step 4: 2763 + 921 = 3684
Step 5: 3684 + 921 = 4605
Step 6: 4605 + 921 = 5526
Step 7: 5526 + 921 = 6447
Step 8: 6447 + 921 = 7368
Step 9: 7368 + 921 = 8289
Step 10: 8289 + 921 = 9210
Step 11: 9210 + 921 = 10131
Step 12: 10131 + 921 = 11052
Step 13: 11052 + 921 = 11973
Step 14: 11973 + 921 = 12894
Step 15: 12894 + 921 = 13815

So, 921*15 = 13815