Answer
$\sqrt{7x}$
Work Step by Step
$\sqrt{63x} - \sqrt{28x}$
By the Product Rule for Square Roots
$=(\sqrt{9} \cdot \sqrt{7x}) - (\sqrt{4} \cdot \sqrt{7x})$
$=3\sqrt{7x} - 2\sqrt{7x}$
By the Distributive Property
$=(3-2)\sqrt{7x}$
$=\sqrt{7x}$