Alright, let's solve this math problem step by step. First, we have the number 158. 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 158 and add it to itself 21 times.
Step 1: 0 + 158 = 158
Step 2: 158 + 158 = 316
Step 3: 316 + 158 = 474
Step 4: 474 + 158 = 632
Step 5: 632 + 158 = 790
Step 6: 790 + 158 = 948
Step 7: 948 + 158 = 1106
Step 8: 1106 + 158 = 1264
Step 9: 1264 + 158 = 1422
Step 10: 1422 + 158 = 1580
Step 11: 1580 + 158 = 1738
Step 12: 1738 + 158 = 1896
Step 13: 1896 + 158 = 2054
Step 14: 2054 + 158 = 2212
Step 15: 2212 + 158 = 2370
Step 16: 2370 + 158 = 2528
Step 17: 2528 + 158 = 2686
Step 18: 2686 + 158 = 2844
Step 19: 2844 + 158 = 3002
Step 20: 3002 + 158 = 3160
Step 21: 3160 + 158 = 3318

So, 158*21 = 3318