OK, let's work through this together. We're starting with 619 and we're going to multiply it by 21, which means we add 619 to itself 21 times.
Step 1: 0 + 619 = 619
Step 2: 619 + 619 = 1238
Step 3: 1238 + 619 = 1857
Step 4: 1857 + 619 = 2476
Step 5: 2476 + 619 = 3095
Step 6: 3095 + 619 = 3714
Step 7: 3714 + 619 = 4333
Step 8: 4333 + 619 = 4952
Step 9: 4952 + 619 = 5571
Step 10: 5571 + 619 = 6190
Step 11: 6190 + 619 = 6809
Step 12: 6809 + 619 = 7428
Step 13: 7428 + 619 = 8047
Step 14: 8047 + 619 = 8666
Step 15: 8666 + 619 = 9285
Step 16: 9285 + 619 = 9904
Step 17: 9904 + 619 = 10523
Step 18: 10523 + 619 = 11142
Step 19: 11142 + 619 = 11761
Step 20: 11761 + 619 = 12380
Step 21: 12380 + 619 = 12999

So, 619*21 = 12999