Alright, let's solve this math problem step by step. First, we have the number 399. Next, we see the multiplication sign, which means we need to multiply something. And what are we multiplying it by? We multiply it by 42. So, we're going to take the number 399 and add it to itself 42 times.
Step 1: 0 + 399 = 399
Step 2: 399 + 399 = 798
Step 3: 798 + 399 = 1197
Step 4: 1197 + 399 = 1596
Step 5: 1596 + 399 = 1995
Step 6: 1995 + 399 = 2394
Step 7: 2394 + 399 = 2793
Step 8: 2793 + 399 = 3192
Step 9: 3192 + 399 = 3591
Step 10: 3591 + 399 = 3990
Step 11: 3990 + 399 = 4389
Step 12: 4389 + 399 = 4788
Step 13: 4788 + 399 = 5187
Step 14: 5187 + 399 = 5586
Step 15: 5586 + 399 = 5985
Step 16: 5985 + 399 = 6384
Step 17: 6384 + 399 = 6783
Step 18: 6783 + 399 = 7182
Step 19: 7182 + 399 = 7581
Step 20: 7581 + 399 = 7980
Step 21: 7980 + 399 = 8379
Step 22: 8379 + 399 = 8778
Step 23: 8778 + 399 = 9177
Step 24: 9177 + 399 = 9576
Step 25: 9576 + 399 = 9975
Step 26: 9975 + 399 = 10374
Step 27: 10374 + 399 = 10773
Step 28: 10773 + 399 = 11172
Step 29: 11172 + 399 = 11571
Step 30: 11571 + 399 = 11970
Step 31: 11970 + 399 = 12369
Step 32: 12369 + 399 = 12768
Step 33: 12768 + 399 = 13167
Step 34: 13167 + 399 = 13566
Step 35: 13566 + 399 = 13965
Step 36: 13965 + 399 = 14364
Step 37: 14364 + 399 = 14763
Step 38: 14763 + 399 = 15162
Step 39: 15162 + 399 = 15561
Step 40: 15561 + 399 = 15960
Step 41: 15960 + 399 = 16359
Step 42: 16359 + 399 = 16758

So, 399*42 = 16758