$$ Total\ students=20 $$

$$ Math\ club=13 \ members $$

$$ Drama\ club=11 \ members $$

As 9 students are members of both clubs. So 13-9=4 of them are members of the math club only and 11-9=2 are members of the Drama club only.

$$ 20-(9+4+2)=20-15=5 $$

5 students are not members of either club.