OK, let's work through this together. We're starting with 773 and we're going to multiply it by 21, which means we add 773 to itself 21 times.
Step 1: 0 + 773 = 773
Step 2: 773 + 773 = 1546
Step 3: 1546 + 773 = 2319
Step 4: 2319 + 773 = 3092
Step 5: 3092 + 773 = 3865
Step 6: 3865 + 773 = 4638
Step 7: 4638 + 773 = 5411
Step 8: 5411 + 773 = 6184
Step 9: 6184 + 773 = 6957
Step 10: 6957 + 773 = 7730
Step 11: 7730 + 773 = 8503
Step 12: 8503 + 773 = 9276
Step 13: 9276 + 773 = 10049
Step 14: 10049 + 773 = 10822
Step 15: 10822 + 773 = 11595
Step 16: 11595 + 773 = 12368
Step 17: 12368 + 773 = 13141
Step 18: 13141 + 773 = 13914
Step 19: 13914 + 773 = 14687
Step 20: 14687 + 773 = 15460
Step 21: 15460 + 773 = 16233

So, 773*21 = 16233