cosine of -120 is in the third quadrant, at 240 degrees ccw from 0 or 120 cw from 0. at this point it is 60 degrees from the third quadrant reference angle of 180. cosine is negative in the third quadrant so the answer is -cos 60.
Just remember "All Students Take Calculus." First quadrant- All are (+). Second quadrant- sine is (+). Third quadrant- tangent is (+). Fourth quadrant- cosine is (+). Using this, you can answer basically any questions related to trig.
