Converting to a mixed number is the same as dividing and leaving the remainder.
Say we had 5/2...we would divide 5 by 2 and end up with 2 r1, right?
So 5/2 = 2 1/2
As for 39/3...we divide 39 by 3...and get exactly 13! There's no remainder, so you don't even need any additional fractions.