#### Answer

$-6\sqrt{10}$

#### Work Step by Step

$\bf{\text{Solution Outline:}}$
To simplify the given radical expression, $
6\sqrt{10}-12\sqrt{10}
,$ combine the like terms.
$\bf{\text{Solution Details:}}$
By combining like terms, the expression above is equivalent to
\begin{array}{l}\require{cancel}
(6-12)\sqrt{10}
\\\\=
-6\sqrt{10}
.\end{array}