OK, let's crack this. We're given 643 and our task is to multiply it by 38. Essentially, we'll be adding 643 to itself 38 times.
Step 1: 0 + 643 = 643
Step 2: 643 + 643 = 1286
Step 3: 1286 + 643 = 1929
Step 4: 1929 + 643 = 2572
Step 5: 2572 + 643 = 3215
Step 6: 3215 + 643 = 3858
Step 7: 3858 + 643 = 4501
Step 8: 4501 + 643 = 5144
Step 9: 5144 + 643 = 5787
Step 10: 5787 + 643 = 6430
Step 11: 6430 + 643 = 7073
Step 12: 7073 + 643 = 7716
Step 13: 7716 + 643 = 8359
Step 14: 8359 + 643 = 9002
Step 15: 9002 + 643 = 9645
Step 16: 9645 + 643 = 10288
Step 17: 10288 + 643 = 10931
Step 18: 10931 + 643 = 11574
Step 19: 11574 + 643 = 12217
Step 20: 12217 + 643 = 12860
Step 21: 12860 + 643 = 13503
Step 22: 13503 + 643 = 14146
Step 23: 14146 + 643 = 14789
Step 24: 14789 + 643 = 15432
Step 25: 15432 + 643 = 16075
Step 26: 16075 + 643 = 16718
Step 27: 16718 + 643 = 17361
Step 28: 17361 + 643 = 18004
Step 29: 18004 + 643 = 18647
Step 30: 18647 + 643 = 19290
Step 31: 19290 + 643 = 19933
Step 32: 19933 + 643 = 20576
Step 33: 20576 + 643 = 21219
Step 34: 21219 + 643 = 21862
Step 35: 21862 + 643 = 22505
Step 36: 22505 + 643 = 23148
Step 37: 23148 + 643 = 23791
Step 38: 23791 + 643 = 24434

So, 643*38 = 24434