Answer
$x^2\sqrt{3x}$
Work Step by Step
Recall the property (pg. 369):
$\dfrac{\sqrt[n]{a}}{\sqrt[n]{b}}=\sqrt[n]{\dfrac{a}{b}}$ (if $\sqrt[n]{a}$ and $\sqrt[n]{b}$ are real numbers and $b\ne 0$)
We use this property to simplify the given expression and obtain:
$=\sqrt{\dfrac{21x^{10}}{7x^5}}$
$=\sqrt{3x^5}$
$=\sqrt{x^4\cdot 3x}$
$=\sqrt{x^4} \cdot \sqrt[2]{3x}$
$=x^2\sqrt{3x}$