Alright, let's solve this math problem step by step. First, we have the number 21. Next, we see the multiplication sign, which means we need to multiply something. And what are we multiplying it by? We multiply it by 10. So, we're going to take the number 21 and add it to itself 10 times.
Step 1: 0 + 21 = 21
Step 2: 21 + 21 = 42
Step 3: 42 + 21 = 63
Step 4: 63 + 21 = 84
Step 5: 84 + 21 = 105
Step 6: 105 + 21 = 126
Step 7: 126 + 21 = 147
Step 8: 147 + 21 = 168
Step 9: 168 + 21 = 189
Step 10: 189 + 21 = 210

So, 21*10 = 210