#### Answer

$\dfrac{7}{\sqrt{35}+\sqrt {21}}$

#### Work Step by Step

Simplify. $ \dfrac{\sqrt 7}{\sqrt 5+\sqrt 3}$
Now, $\dfrac{\sqrt 7}{\sqrt 5+\sqrt 3}=\dfrac{\sqrt 7}{\sqrt 5+\sqrt 3} \times \dfrac{\sqrt 7}{\sqrt 7}$
or, $=\dfrac{(\sqrt7)^2}{\sqrt 7(\sqrt 5+\sqrt 3)}$
$\implies \dfrac{7}{\sqrt{35}+\sqrt {21}}$