Let's roll up our sleeves and solve this. We have 173 and we're going to multiply it by 21, essentially adding 173 to itself 21 times.
Step 1: 0 + 173 = 173
Step 2: 173 + 173 = 346
Step 3: 346 + 173 = 519
Step 4: 519 + 173 = 692
Step 5: 692 + 173 = 865
Step 6: 865 + 173 = 1038
Step 7: 1038 + 173 = 1211
Step 8: 1211 + 173 = 1384
Step 9: 1384 + 173 = 1557
Step 10: 1557 + 173 = 1730
Step 11: 1730 + 173 = 1903
Step 12: 1903 + 173 = 2076
Step 13: 2076 + 173 = 2249
Step 14: 2249 + 173 = 2422
Step 15: 2422 + 173 = 2595
Step 16: 2595 + 173 = 2768
Step 17: 2768 + 173 = 2941
Step 18: 2941 + 173 = 3114
Step 19: 3114 + 173 = 3287
Step 20: 3287 + 173 = 3460
Step 21: 3460 + 173 = 3633

So, 173*21 = 3633