Answer
$(7x)^{\frac{3}{2}}$
Work Step by Step
Recall the rational exponent property (pg. 382):
$a^{\frac{m}{n}}=\sqrt[n]{a^m}=(\sqrt[n]{a})^m$
Applying this property ($m=3$, $n=2$, $a=7x$), we get:
$\left(\sqrt{7x}\right)^3=(7x)^{\frac{3}{2}}$