OK, let's work through this together. We're starting with 664 and we're going to multiply it by 21, which means we add 664 to itself 21 times.
Step 1: 0 + 664 = 664
Step 2: 664 + 664 = 1328
Step 3: 1328 + 664 = 1992
Step 4: 1992 + 664 = 2656
Step 5: 2656 + 664 = 3320
Step 6: 3320 + 664 = 3984
Step 7: 3984 + 664 = 4648
Step 8: 4648 + 664 = 5312
Step 9: 5312 + 664 = 5976
Step 10: 5976 + 664 = 6640
Step 11: 6640 + 664 = 7304
Step 12: 7304 + 664 = 7968
Step 13: 7968 + 664 = 8632
Step 14: 8632 + 664 = 9296
Step 15: 9296 + 664 = 9960
Step 16: 9960 + 664 = 10624
Step 17: 10624 + 664 = 11288
Step 18: 11288 + 664 = 11952
Step 19: 11952 + 664 = 12616
Step 20: 12616 + 664 = 13280
Step 21: 13280 + 664 = 13944

So, 664*21 = 13944