Chapter 10 - Exponents and Radicals - 10.4 Dividing Radical Expressions - 10.4 Exercise Set - Page 654: 69

$\dfrac{7x}{\sqrt{21xy}}$

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}