OK, let's work through this together. We're starting with 749 and we're going to multiply it by 21, which means we add 749 to itself 21 times.
Step 1: 0 + 749 = 749
Step 2: 749 + 749 = 1498
Step 3: 1498 + 749 = 2247
Step 4: 2247 + 749 = 2996
Step 5: 2996 + 749 = 3745
Step 6: 3745 + 749 = 4494
Step 7: 4494 + 749 = 5243
Step 8: 5243 + 749 = 5992
Step 9: 5992 + 749 = 6741
Step 10: 6741 + 749 = 7490
Step 11: 7490 + 749 = 8239
Step 12: 8239 + 749 = 8988
Step 13: 8988 + 749 = 9737
Step 14: 9737 + 749 = 10486
Step 15: 10486 + 749 = 11235
Step 16: 11235 + 749 = 11984
Step 17: 11984 + 749 = 12733
Step 18: 12733 + 749 = 13482
Step 19: 13482 + 749 = 14231
Step 20: 14231 + 749 = 14980
Step 21: 14980 + 749 = 15729

So, 749*21 = 15729