I am trying to find the derivative of $\sqrt{9-x}$ using the definition of a derivative
$\lim_{h\to 0} \frac {f(a+h)-f(a)}{h} $
$\lim_{h\to 0} \frac {\sqrt{9-(a+h)}-\sqrt{9-a}}{h} $
So to simplify I multiply by the conjugate
$\lim_{h\to0} \frac {\sqrt{9-(a+h)}-\sqrt{9-a}}{h}\cdot \frac{ \sqrt{9-(a+h)}+ \sqrt{9-a}}{\sqrt{9-(a+h)}+\sqrt{9-a}}$
which gives me
$\frac {-2a-h}{h(\sqrt{9-(a+h)}+\sqrt{9-a})}$
I have no idea what to do from here, obviously I can easily get the derivative using other methods but with this one I have no idea how to proceed.