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 $$f$$?