Express in the form
Square both sides: .
And we have: .
Equate coefficients: .
. . And the rational roots are: .
There are two solutions: .
See the in the denominator? The sign indicates that the number that follows has to be the opposite of whatever sign the number after the is. If the denominator is +6, then it simplifies to -2, and if the denominator is -6, it simplifies to +2.
The isn't used as often as . If you know trig, then you may have seen the sum/difference identity for cosine combined into one identity like this:
Of course, the two identities are these:
Note how the sign changes on the right side. That's the reason we use the sign.