Well, we know that 1, 2, and -2 are zeros. So the 4th zero is real as well. (If it were complex then there would be a fifth zero: the complex conjugate of the fourth zero.) So the polynomial is of the form:

where a is the fourth zero, currently unknown.

We also know that

From our polynomial we know that

so .

(The condition gives us the same value.)

Thus

-Dan