Answer
$\dfrac{6}{\sqrt{15y}}$
Work Step by Step
Rationalizing the numerator, the given expression, $
\dfrac{\sqrt{12}}{\sqrt{5y}}
,$ is equivalent to
\begin{array}{l}
\dfrac{\sqrt{12}}{\sqrt{5y}}\cdot\dfrac{\sqrt{3}}{\sqrt{3}}
\\\\=
\dfrac{\sqrt{36}}{\sqrt{15y}}
\\\\=
\dfrac{6}{\sqrt{15y}}
.\end{array}