Answer
$-14\sqrt{7}$.
Work Step by Step
The given expression is
$=4\sqrt{7}-6\sqrt{63}$
Factor as square terms.
$=4\sqrt{7}-6\sqrt{9\cdot 7}$
Use $9=3^2$.
$=4\sqrt{7}-6\sqrt{3^2\cdot 7}$
Use product property of square roots.
$=4\sqrt{7}-6\sqrt{3^2}\cdot \sqrt{7}$
Simplify.
$=4\sqrt{7}-6\cdot 3 \sqrt{7}$
$=4\sqrt{7}-18 \sqrt{7}$
Use distributive property.
$=(4-18)\sqrt{7}$
Subtract.
$=-14\sqrt{7}$.
