Alright, let's solve this math problem step by step. First, we have the number 343. 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 343 and add it to itself 21 times.
Step 1: 0 + 343 = 343
Step 2: 343 + 343 = 686
Step 3: 686 + 343 = 1029
Step 4: 1029 + 343 = 1372
Step 5: 1372 + 343 = 1715
Step 6: 1715 + 343 = 2058
Step 7: 2058 + 343 = 2401
Step 8: 2401 + 343 = 2744
Step 9: 2744 + 343 = 3087
Step 10: 3087 + 343 = 3430
Step 11: 3430 + 343 = 3773
Step 12: 3773 + 343 = 4116
Step 13: 4116 + 343 = 4459
Step 14: 4459 + 343 = 4802
Step 15: 4802 + 343 = 5145
Step 16: 5145 + 343 = 5488
Step 17: 5488 + 343 = 5831
Step 18: 5831 + 343 = 6174
Step 19: 6174 + 343 = 6517
Step 20: 6517 + 343 = 6860
Step 21: 6860 + 343 = 7203

So, 343*21 = 7203