Answer
$\frac{1}{(\sqrt{z-3}+\sqrt z)}$
Work Step by Step
$\frac{\sqrt{z-3}-\sqrt z}{-3}=\frac{(\sqrt{z-3}-\sqrt z)(\sqrt{z-3}+\sqrt z)}{-3(\sqrt{z-3}+\sqrt z)}=\frac{z-3-z}{-3(\sqrt{z-3}+\sqrt z)}=\frac{-3}{-3(\sqrt{z-3}+\sqrt z)}=\frac{1}{(\sqrt{z-3}+\sqrt z)}$
