since 8/26 want to play soccer and 12/26 want to play basketball, you would add 8/26 and 12/26 to get 20/26, then you would subtract 20/26 from 26/26, leaving you with 6/26 of the students wanting to play baseball.
note: if the question wants you to simplify you would divide both your numerator and your denominator by 2 to get 3/13 as your final simplified answer.