OK, let's crack this. We're given 637 and our task is to multiply it by 21. Essentially, we'll be adding 637 to itself 21 times.
Step 1: 0 + 637 = 637
Step 2: 637 + 637 = 1274
Step 3: 1274 + 637 = 1911
Step 4: 1911 + 637 = 2548
Step 5: 2548 + 637 = 3185
Step 6: 3185 + 637 = 3822
Step 7: 3822 + 637 = 4459
Step 8: 4459 + 637 = 5096
Step 9: 5096 + 637 = 5733
Step 10: 5733 + 637 = 6370
Step 11: 6370 + 637 = 7007
Step 12: 7007 + 637 = 7644
Step 13: 7644 + 637 = 8281
Step 14: 8281 + 637 = 8918
Step 15: 8918 + 637 = 9555
Step 16: 9555 + 637 = 10192
Step 17: 10192 + 637 = 10829
Step 18: 10829 + 637 = 11466
Step 19: 11466 + 637 = 12103
Step 20: 12103 + 637 = 12740
Step 21: 12740 + 637 = 13377

So, 637*21 = 13377