Answer
$-7\sqrt5$
Work Step by Step
$3\sqrt{45}-8\sqrt{20}\longrightarrow$ factor the perfect squares out of 45 and 20
=$3\sqrt{9\times5}-8\sqrt{4\times5}\longrightarrow$ multiplication property of square roots
=$3(\sqrt{9})(\sqrt5)-8(\sqrt{4})(\sqrt5)\longrightarrow$ simplify
=$3(3)(\sqrt5)-8(2)(\sqrt5)\longrightarrow$ multiply
=$9\sqrt5-16\sqrt5\longrightarrow$ use distributive property
=$\sqrt5(9-16)\longrightarrow$ simplify
=$-7\sqrt5$
