OK, let's work through this together. We're starting with 621 and we're going to multiply it by 39, which means we add 621 to itself 39 times.
Step 1: 0 + 621 = 621
Step 2: 621 + 621 = 1242
Step 3: 1242 + 621 = 1863
Step 4: 1863 + 621 = 2484
Step 5: 2484 + 621 = 3105
Step 6: 3105 + 621 = 3726
Step 7: 3726 + 621 = 4347
Step 8: 4347 + 621 = 4968
Step 9: 4968 + 621 = 5589
Step 10: 5589 + 621 = 6210
Step 11: 6210 + 621 = 6831
Step 12: 6831 + 621 = 7452
Step 13: 7452 + 621 = 8073
Step 14: 8073 + 621 = 8694
Step 15: 8694 + 621 = 9315
Step 16: 9315 + 621 = 9936
Step 17: 9936 + 621 = 10557
Step 18: 10557 + 621 = 11178
Step 19: 11178 + 621 = 11799
Step 20: 11799 + 621 = 12420
Step 21: 12420 + 621 = 13041
Step 22: 13041 + 621 = 13662
Step 23: 13662 + 621 = 14283
Step 24: 14283 + 621 = 14904
Step 25: 14904 + 621 = 15525
Step 26: 15525 + 621 = 16146
Step 27: 16146 + 621 = 16767
Step 28: 16767 + 621 = 17388
Step 29: 17388 + 621 = 18009
Step 30: 18009 + 621 = 18630
Step 31: 18630 + 621 = 19251
Step 32: 19251 + 621 = 19872
Step 33: 19872 + 621 = 20493
Step 34: 20493 + 621 = 21114
Step 35: 21114 + 621 = 21735
Step 36: 21735 + 621 = 22356
Step 37: 22356 + 621 = 22977
Step 38: 22977 + 621 = 23598
Step 39: 23598 + 621 = 24219

So, 621*39 = 24219