The question is this:
Question: Identify a suitable universe of discourse such that the predicate z2 + 4 = 0 has existential import.
Now if you try to factor this equation, you will see that it is not possible.
If you try to solve for z, what you get is this:
z2 - 4i2
(z + 2i) (z - 2i)
z = +-2i
So The only suitable universe of discourse is complex numbers.
Is my answer correct?
Edit: In set theory notation, it would look something like this ᗄx∈ in the complex number set. Please also check that my notation is correct.