#### Answer

$\frac{10x^2y}{14x^3y}$

#### Work Step by Step

For $\frac{5}{7x} = \frac{}{14x^3y}$,
$\frac{5}{7x} = \frac{(5)(2x^2y)}{(7x)(2x^2y)} = \frac{10x^2y}{14x^3y}$,
The equivalent rational expression of $\frac{5}{7x}$ with the given denominator is $\frac{10x^2y}{14x^3y}$.