Answer
$\dfrac{3}{\sqrt{21}}$
Work Step by Step
Rationalizing the numerator of $
\sqrt{\dfrac{3}{7}}
$ results to
\begin{array}{l}
\sqrt{\dfrac{3}{7}}
\cdot
\sqrt{\dfrac{3}{3}}
\\\\=
\sqrt{\dfrac{9}{21}}
\\\\=
\dfrac{3}{\sqrt{21}}
.\end{array}