Sure thing! We've got 252 and we're gonna multiply it by 21. That's the same as adding 252 to itself 21 times.
Step 1: 0 + 252 = 252
Step 2: 252 + 252 = 504
Step 3: 504 + 252 = 756
Step 4: 756 + 252 = 1008
Step 5: 1008 + 252 = 1260
Step 6: 1260 + 252 = 1512
Step 7: 1512 + 252 = 1764
Step 8: 1764 + 252 = 2016
Step 9: 2016 + 252 = 2268
Step 10: 2268 + 252 = 2520
Step 11: 2520 + 252 = 2772
Step 12: 2772 + 252 = 3024
Step 13: 3024 + 252 = 3276
Step 14: 3276 + 252 = 3528
Step 15: 3528 + 252 = 3780
Step 16: 3780 + 252 = 4032
Step 17: 4032 + 252 = 4284
Step 18: 4284 + 252 = 4536
Step 19: 4536 + 252 = 4788
Step 20: 4788 + 252 = 5040
Step 21: 5040 + 252 = 5292

So, 252*21 = 5292