We have

Isolate the square root (I'll explain why in a minute):

A square root is always positive. So, if
equals the square root of something, it must be positive. So, we must impose

So, we will only accept solutions which are greater than or equal to -2. With that said, let's square both sides:

We can simplify z^2, since it appears in both sides:

subtract 4 from both sides:

Finally, divide both sides by 4:

This number is not greater than -2, so we can't accept the solution.