Express in the form
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.