Alright, let's solve this math problem step by step. First, we have the number 790. 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 790 and add it to itself 21 times.
Step 1: 0 + 790 = 790
Step 2: 790 + 790 = 1580
Step 3: 1580 + 790 = 2370
Step 4: 2370 + 790 = 3160
Step 5: 3160 + 790 = 3950
Step 6: 3950 + 790 = 4740
Step 7: 4740 + 790 = 5530
Step 8: 5530 + 790 = 6320
Step 9: 6320 + 790 = 7110
Step 10: 7110 + 790 = 7900
Step 11: 7900 + 790 = 8690
Step 12: 8690 + 790 = 9480
Step 13: 9480 + 790 = 10270
Step 14: 10270 + 790 = 11060
Step 15: 11060 + 790 = 11850
Step 16: 11850 + 790 = 12640
Step 17: 12640 + 790 = 13430
Step 18: 13430 + 790 = 14220
Step 19: 14220 + 790 = 15010
Step 20: 15010 + 790 = 15800
Step 21: 15800 + 790 = 16590

So, 790*21 = 16590