#### Answer

$\frac{7}{\sqrt {21}}$

#### Work Step by Step

$\sqrt \frac{7}{3}$
=$\frac{\sqrt 7}{\sqrt 3}$
=$\frac{\sqrt 7}{\sqrt 3}\times\frac{\sqrt 7}{\sqrt 7}$
=$\frac{\sqrt 7\times\sqrt 7}{\sqrt 3\times\sqrt 7}$
=$\frac{7}{\sqrt {3\times7}}$
=$\frac{7}{\sqrt {21}}$