OK, let's crack this. We're given 321 and our task is to multiply it by 21. Essentially, we'll be adding 321 to itself 21 times.
Step 1: 0 + 321 = 321
Step 2: 321 + 321 = 642
Step 3: 642 + 321 = 963
Step 4: 963 + 321 = 1284
Step 5: 1284 + 321 = 1605
Step 6: 1605 + 321 = 1926
Step 7: 1926 + 321 = 2247
Step 8: 2247 + 321 = 2568
Step 9: 2568 + 321 = 2889
Step 10: 2889 + 321 = 3210
Step 11: 3210 + 321 = 3531
Step 12: 3531 + 321 = 3852
Step 13: 3852 + 321 = 4173
Step 14: 4173 + 321 = 4494
Step 15: 4494 + 321 = 4815
Step 16: 4815 + 321 = 5136
Step 17: 5136 + 321 = 5457
Step 18: 5457 + 321 = 5778
Step 19: 5778 + 321 = 6099
Step 20: 6099 + 321 = 6420
Step 21: 6420 + 321 = 6741

So, 321*21 = 6741