OK, let's crack this. We're given 508 and our task is to multiply it by 21. Essentially, we'll be adding 508 to itself 21 times.
Step 1: 0 + 508 = 508
Step 2: 508 + 508 = 1016
Step 3: 1016 + 508 = 1524
Step 4: 1524 + 508 = 2032
Step 5: 2032 + 508 = 2540
Step 6: 2540 + 508 = 3048
Step 7: 3048 + 508 = 3556
Step 8: 3556 + 508 = 4064
Step 9: 4064 + 508 = 4572
Step 10: 4572 + 508 = 5080
Step 11: 5080 + 508 = 5588
Step 12: 5588 + 508 = 6096
Step 13: 6096 + 508 = 6604
Step 14: 6604 + 508 = 7112
Step 15: 7112 + 508 = 7620
Step 16: 7620 + 508 = 8128
Step 17: 8128 + 508 = 8636
Step 18: 8636 + 508 = 9144
Step 19: 9144 + 508 = 9652
Step 20: 9652 + 508 = 10160
Step 21: 10160 + 508 = 10668

So, 508*21 = 10668