What is the square root of - 9 in simplest form

It is 3i.

The square root of - 9 is the square root of - 1 times the square root of 9.

So: the square root of 9 is 3, and the square root of - 1 is called i (this doesn't actually exist, it's just imaginary).

Then, the square root of - 9 is 3i.