2wholes and 3/12 + 3/? = 2 wholes and 5/8 .what is the missing number ?

Basically you know that the 2 wholes from both sides cancel out.

Set up the equation, 5/8 = 3/12 + 3/x

3/x =  5/8 - 3/12

Set them to have a common denominator, 3/x = 15/24 - 6/24

3/x = 9/24

Simplify 9/24 and you will get 3/8

3/x = 3/8

Obviously this means x = 8