#### Answer

$\frac{6}{\sqrt {2y}}$

#### Work Step by Step

$\sqrt \frac{18}{y}$
=$\frac{\sqrt {18}}{\sqrt y}$
=$\frac{\sqrt {2\times9}}{\sqrt y}$
=$\frac{3\sqrt {2}}{\sqrt y}$
=$\frac{3\sqrt {2}}{\sqrt y}\times\frac{3\sqrt {2}}{3\sqrt {2}}$
=$\frac{3\sqrt {2}\times3\sqrt {2}}{\sqrt y\times3\sqrt {2}}$
=$\frac{9\times\sqrt {2}\times\sqrt {2}}{3\sqrt {2y}}$
=$\frac{9\times2}{3\sqrt {2y}}$
=$\frac{6}{\sqrt {2y}}$