Answer
$9\sqrt{10}$
Work Step by Step
$-6\sqrt{10}+5\sqrt{90}\longrightarrow$ factor the perfect square out of 90
=$-6\sqrt{10}+5\sqrt{9\times10}\longrightarrow$ multiplication property of square roots
=$-6\sqrt{10}+5(\sqrt{9})(\sqrt{10})\longrightarrow$ simplify
=$-6\sqrt{10}+5(3)(\sqrt{10})\longrightarrow$ multiply
=$-6\sqrt{10}+15\sqrt{10}\longrightarrow$ use distributive property
=$\sqrt{10}(-6+15)\longrightarrow$ simplify
=$9\sqrt{10}$
