Answer
$-2\sqrt 5$
Work Step by Step
$\sqrt {20}+\sqrt {45}-7\sqrt {5}$
=$\sqrt {4\times5}+\sqrt {9\times5}-7\sqrt {5}$
=$(\sqrt {4}\times\sqrt {5})+(\sqrt 9\times\sqrt 5)-7\sqrt {5}$
=$(2\sqrt {5})+(3\sqrt 5)-7\sqrt {5}$
=$(2+3-7)\sqrt 5$
=$-2\sqrt 5$
