$\displaystyle \frac{\sqrt{x^2-9}-[x(\frac{1}{2}(x^2-9)(2x)]}\sqrt({x^2-9})^2$

I was able to clean this up:

$\displaystyle \frac{\sqrt{x^2-9}-\frac{x^2}{\sqrt{x^2-9}}}\sqrt({x^2-9})^2$

I don't know how to proceed any further. I know the simplified answer is:

$\displaystyle \frac{-9}{(x^2-9)^\frac{3}{2}}$

But I really don't know how to get there.