Answer
$-|x|\sqrt{5x}$
Work Step by Step
Using the laws of exponents, the given expression, $
\sqrt{125x^3}-3\sqrt{20x^3}
,$ simplifies to
\begin{array}{l}\require{cancel}
\sqrt{25x^2\cdot5x}-3\sqrt{4x^2\cdot5x}
\\\\=
\sqrt{(5x)^2\cdot5x}-3\sqrt{(2x)^2\cdot5x}
\\\\=
|5x|\sqrt{5x}-3\cdot|2x|\sqrt{5x}
\\\\=
5|x|\sqrt{5x}-3\cdot2|x|\sqrt{5x}
\\\\=
5|x|\sqrt{5x}-6|x|\sqrt{5x}
\\\\=
(5-6)|x|\sqrt{5x}
\\\\=
-|x|\sqrt{5x}
.\end{array}
