Alright, let's solve this math problem step by step. First, we have the number 12. 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 12 and add it to itself 21 times.
Step 1: 0 + 12 = 12
Step 2: 12 + 12 = 24
Step 3: 24 + 12 = 36
Step 4: 36 + 12 = 48
Step 5: 48 + 12 = 60
Step 6: 60 + 12 = 72
Step 7: 72 + 12 = 84
Step 8: 84 + 12 = 96
Step 9: 96 + 12 = 108
Step 10: 108 + 12 = 120
Step 11: 120 + 12 = 132
Step 12: 132 + 12 = 144
Step 13: 144 + 12 = 156
Step 14: 156 + 12 = 168
Step 15: 168 + 12 = 180
Step 16: 180 + 12 = 192
Step 17: 192 + 12 = 204
Step 18: 204 + 12 = 216
Step 19: 216 + 12 = 228
Step 20: 228 + 12 = 240
Step 21: 240 + 12 = 252

So, 12*21 = 252