Answer
0
Work Step by Step
$3\sqrt7-\sqrt{63}\longrightarrow$ factor the perfect square out of 63
=$3\sqrt7-\sqrt{9\times7}\longrightarrow$ multiplication property of square roots
=$3\sqrt7-(\sqrt{9}\times\sqrt7)\longrightarrow$ simplify
=$3\sqrt7-3\sqrt7\longrightarrow$ use distributive property
=$\sqrt7(3-3)\longrightarrow$ simplify
=$0\sqrt7\longrightarrow$ the product of any number and 0 is 0
=0
