#### Answer

$-7\sqrt 5$

#### Work Step by Step

You are given $3\sqrt 45$ $-$ $8\sqrt 20$.Simplify:
Factor perfect squares out of the radicands:
$3 \sqrt 9*5$ $-$ $8 \sqrt 4*5$
Multiplication property:
$3 (\sqrt 9) ({\sqrt 5})$ $-$ $8 (\sqrt 4) ({\sqrt 5})$
$3(3) (\sqrt 5)$ $-$ $8(2) (\sqrt 5)$
$9\sqrt 5$ $-$ $16\sqrt 5$
$\sqrt 5(9-16)$
=$-7\sqrt 5$