OK, let's crack this. We're given 172 and our task is to multiply it by 21. Essentially, we'll be adding 172 to itself 21 times.
Step 1: 0 + 172 = 172
Step 2: 172 + 172 = 344
Step 3: 344 + 172 = 516
Step 4: 516 + 172 = 688
Step 5: 688 + 172 = 860
Step 6: 860 + 172 = 1032
Step 7: 1032 + 172 = 1204
Step 8: 1204 + 172 = 1376
Step 9: 1376 + 172 = 1548
Step 10: 1548 + 172 = 1720
Step 11: 1720 + 172 = 1892
Step 12: 1892 + 172 = 2064
Step 13: 2064 + 172 = 2236
Step 14: 2236 + 172 = 2408
Step 15: 2408 + 172 = 2580
Step 16: 2580 + 172 = 2752
Step 17: 2752 + 172 = 2924
Step 18: 2924 + 172 = 3096
Step 19: 3096 + 172 = 3268
Step 20: 3268 + 172 = 3440
Step 21: 3440 + 172 = 3612

So, 172*21 = 3612