Question

nothing

Answer 1

square root of -1 is i. So you can take that out of the square root.
You are left with 72. Factor 72 so that it is a product of a perfect square and another number. Can you do that?

Answer 2

\[\sqrt{72}=\sqrt{9} \times \sqrt{8} = 3 \times \sqrt{4} \times \sqrt{2} = 6\sqrt{2}\]\[\sqrt{-72}=6i\sqrt{2}\]