Complex numbers root problems

Homework Statement

What is the square root of z=-9

The Attempt at a Solution

Is it possible of me to not use De'Moivre's theorem to solve this question??
Solution :
z= $\sqrt{-9}$
z=$\sqrt{9}$ x $\sqrt{}-1$
z=$\pm3$ i

Will this method be acceptable?? Is this still under the topic complex roots?