Answer
$ \frac{2x+h-2\sqrt {x^2+xh}}{h}$
Work Step by Step
$\frac{\sqrt {x+h}-\sqrt x}{\sqrt {x+h}+\sqrt x}=\frac{\sqrt {x+h}-\sqrt x}{\sqrt {x+h}+\sqrt x} \times\frac{\sqrt {x+h}-\sqrt x}{\sqrt {x+h}-\sqrt x}=\frac{x+h-2\sqrt {x+h}\sqrt x +x}{x+h-x}=\frac{2x+h-2\sqrt {x^2+xh}}{h}$
