Answer
-27
Work Step by Step
$-9^{\frac{3}{2}}=-(\sqrt (9^{3}))=-(\sqrt (9\times9\times9))=-(\sqrt 9\times\sqrt 9\times\sqrt 9))=-(3\times3\times3)=-(27)=-27$
We know that $\sqrt 9=3$, because $3^{2}=9$.
In this case, we are finding the opposite of $\sqrt (9^{3})$.