Answer
$-3\sqrt7$
Work Step by Step
$\sqrt{28}-5\sqrt{7}\longrightarrow$ factor the perfect square out of 28
=$\sqrt{4\times7}-5\sqrt7\longrightarrow$ multiplication property of square roots
=$(\sqrt{4})(\sqrt7)-5\sqrt{7}\longrightarrow$ simplify
=$2\sqrt7-5\sqrt7\longrightarrow$ use distributive property
=$\sqrt7(2-5)\longrightarrow$ simplify
=$-3\sqrt7$
