Answer
$\sqrt6$
Work Step by Step
$3\sqrt6-\sqrt{24}\longrightarrow$ factor out the perfect square from 24
$3\sqrt6-\sqrt{4\times6}\longrightarrow$ multiplication property of square roots
=$3\sqrt6-(\sqrt4\times\sqrt6)\longrightarrow$ simplify
=$3\sqrt6-2\sqrt6\longrightarrow$ use distributive property
=$\sqrt6(3-2)\longrightarrow$ simplify
=$\sqrt6$