Problem Solving

I came across this in the manhattan guide,

(sqrt(x^2 - 2)) - (sqrt(x)) = 0

When you solve it, you get two roots, x = -1 and x = 2

However the answer is only 2 because if you sub in -1 into the equation it will be an imaginary number.

If the GMAT posed this type of question, asking for the roots, would the roots be 2 and -1 or would it be just 2 because they're not really testing your knowledge on imaginary numbers, just the fact that there are imaginary numbers?

Thanks.