Finding a Critical Number

Let the function $f$ be defined by $f(x)=\sqrt{x+8}+\frac{1}{\sqrt{x+8}}$ for $x > -8$. The derivative of $f$ is given by $f'(x)=\frac{1}{2\sqrt{x+8}}-\frac{1}{2(x+8)^{3/2}}$. What is the critical number of the function $f$?