Sure thing, let's get straight to it. We start with 804 and we're going to multiply it by 21, which means adding 804 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