Alright, let's solve this math problem step by step. First, we have the number 509. 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 509 and add it to itself 21 times.
Step 1: 0 + 509 = 509
Step 2: 509 + 509 = 1018
Step 3: 1018 + 509 = 1527
Step 4: 1527 + 509 = 2036
Step 5: 2036 + 509 = 2545
Step 6: 2545 + 509 = 3054
Step 7: 3054 + 509 = 3563
Step 8: 3563 + 509 = 4072
Step 9: 4072 + 509 = 4581
Step 10: 4581 + 509 = 5090
Step 11: 5090 + 509 = 5599
Step 12: 5599 + 509 = 6108
Step 13: 6108 + 509 = 6617
Step 14: 6617 + 509 = 7126
Step 15: 7126 + 509 = 7635
Step 16: 7635 + 509 = 8144
Step 17: 8144 + 509 = 8653
Step 18: 8653 + 509 = 9162
Step 19: 9162 + 509 = 9671
Step 20: 9671 + 509 = 10180
Step 21: 10180 + 509 = 10689

So, 509*21 = 10689