Alright, let's solve this math problem step by step. First, we have the number 804. 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 804 and add it to itself 21 times.
Step 1: 0 + 804 = 804
Step 2: 804 + 804 = 1608
Step 3: 1608 + 804 = 2412
Step 4: 2412 + 804 = 3216
Step 5: 3216 + 804 = 4020
Step 6: 4020 + 804 = 4824
Step 7: 4824 + 804 = 5628
Step 8: 5628 + 804 = 6432
Step 9: 6432 + 804 = 7236
Step 10: 7236 + 804 = 8040
Step 11: 8040 + 804 = 8844
Step 12: 8844 + 804 = 9648
Step 13: 9648 + 804 = 10452
Step 14: 10452 + 804 = 11256
Step 15: 11256 + 804 = 12060
Step 16: 12060 + 804 = 12864
Step 17: 12864 + 804 = 13668
Step 18: 13668 + 804 = 14472
Step 19: 14472 + 804 = 15276
Step 20: 15276 + 804 = 16080
Step 21: 16080 + 804 = 16884

So, 804*21 = 16884