Here's one way to reduce the radical:
[tex]\sqrt{216} = \sqrt{36*6}\\\\\sqrt{216} = \sqrt{36}*\sqrt{6}\\\\\sqrt{216} = 6\sqrt{6}\\\\[/tex]
I factored 216 so that one of the factors (36) is a perfect square, and it's the largest such factor possible. Then I used the rule that [tex]\sqrt{x*y} = \sqrt{x}*\sqrt{y}\\\\[/tex] to break up the radicand on the second step.
