Sure thing, let's get straight to it. We start with 399 and we're going to multiply it by 21, which means adding 399 to itself 21 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

So, 399*21 = 8379