Alright, let's solve this math problem step by step. First, we have the number 603. 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 603 and add it to itself 21 times.
Step 1: 0 + 603 = 603
Step 2: 603 + 603 = 1206
Step 3: 1206 + 603 = 1809
Step 4: 1809 + 603 = 2412
Step 5: 2412 + 603 = 3015
Step 6: 3015 + 603 = 3618
Step 7: 3618 + 603 = 4221
Step 8: 4221 + 603 = 4824
Step 9: 4824 + 603 = 5427
Step 10: 5427 + 603 = 6030
Step 11: 6030 + 603 = 6633
Step 12: 6633 + 603 = 7236
Step 13: 7236 + 603 = 7839
Step 14: 7839 + 603 = 8442
Step 15: 8442 + 603 = 9045
Step 16: 9045 + 603 = 9648
Step 17: 9648 + 603 = 10251
Step 18: 10251 + 603 = 10854
Step 19: 10854 + 603 = 11457
Step 20: 11457 + 603 = 12060
Step 21: 12060 + 603 = 12663

So, 603*21 = 12663