I have the problem in the book of simplify:

(-a^{3}b^{3})^{-2/3 }

I get :

______1_______(-a

^{3}b^{3})^{2/3 }

= _______1______

(-a^{2}b^{2})

The books answer is the same except that a^{2}b^{2 }is positive. Where did I go wrong?

Thanks