Answer
$\sqrt7$
Work Step by Step
$2\sqrt{28}-3\sqrt7\longrightarrow$ factor out the perfect square
=$2\sqrt{4\times7}-3\sqrt7\longrightarrow$ multiplication property of square roots
=$2\times\sqrt4\times\sqrt7-3\sqrt7\longrightarrow$ simplify
=$4\sqrt7-3\sqrt7\longrightarrow$ distributive property
=$\sqrt7(4-3)\longrightarrow$ subtract
=$\sqrt7$