Answer
$\dfrac{7x}{\sqrt{21xy}}$
Work Step by Step
Rationalizing the numerator, the given expression, $
\sqrt{\dfrac{7x}{3y}}
,$ is equivalent to
\begin{array}{l}
\sqrt{\dfrac{7x}{3y}\cdot\dfrac{7x}{7x}}
\\\\=
\sqrt{\dfrac{(7x)^2}{21xy}}
\\\\=
\dfrac{7x}{\sqrt{21xy}}
.\end{array}