Answer: 6√2
Step-by-step explanation: The easiest way to do this problem is to factor 72 as 2 · 36 then recognize 36 as a perfect square, 6 · 6.
There is no need to factor further because the 6's pair up.
So a 6 comes out of the radical leaving a 2 inside.
Since the 2 doesn't pair up, it becomes the radicand.
When simplifying radicals using the factor tree,
always be on the lookout for perfect squares.