Answer
$-2\sqrt{5}$
Work Step by Step
Using the properties of radicals, the expression $
\sqrt{20}+\sqrt{45}-7\sqrt{5}
$ simplifies to
\begin{array}{l}
\sqrt{4\cdot5}+\sqrt{9\cdot5}-7\sqrt{5}
\\=
2\sqrt{5}+3\sqrt{5}-7\sqrt{5}
\\=
-2\sqrt{5}
.\end{array}