Let's roll up our sleeves and solve this. We have 999 and we're going to multiply it by 6, essentially adding 999 to itself 6 times.
Step 1: 0 + 999 = 999
Step 2: 999 + 999 = 1998
Step 3: 1998 + 999 = 2997
Step 4: 2997 + 999 = 3996
Step 5: 3996 + 999 = 4995
Step 6: 4995 + 999 = 5994

So, 999*6 = 5994