Answer
$\frac{1}{\sqrt {x+h}+\sqrt x} $
Work Step by Step
Step 1. $f(x+h)=\sqrt {x+h}$
Step 2. $f(x+h)-f(x)=\sqrt {x+h}-\sqrt x$
Step 3. $\frac{f(x+h)-f(x)}{h}=\frac{\sqrt {x+h}-\sqrt x}{h}=\frac{\sqrt {x+h}-\sqrt x}{h}\times\frac{\sqrt {x+h}+\sqrt x}{\sqrt {x+h}+\sqrt x}=\frac{1}{\sqrt {x+h}+\sqrt x} $
