Let's roll up our sleeves and solve this. We have 339 and we're going to multiply it by 21, essentially adding 339 to itself 21 times.
Step 1: 0 + 339 = 339
Step 2: 339 + 339 = 678
Step 3: 678 + 339 = 1017
Step 4: 1017 + 339 = 1356
Step 5: 1356 + 339 = 1695
Step 6: 1695 + 339 = 2034
Step 7: 2034 + 339 = 2373
Step 8: 2373 + 339 = 2712
Step 9: 2712 + 339 = 3051
Step 10: 3051 + 339 = 3390
Step 11: 3390 + 339 = 3729
Step 12: 3729 + 339 = 4068
Step 13: 4068 + 339 = 4407
Step 14: 4407 + 339 = 4746
Step 15: 4746 + 339 = 5085
Step 16: 5085 + 339 = 5424
Step 17: 5424 + 339 = 5763
Step 18: 5763 + 339 = 6102
Step 19: 6102 + 339 = 6441
Step 20: 6441 + 339 = 6780
Step 21: 6780 + 339 = 7119

So, 339*21 = 7119