What is 39/3 as a mixed number?

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.