[tex]2cos(x) + 1 = 0[/tex]
[tex]cos(x) = -\frac{1}{2}[/tex]
The basic angle for 1/2 is 60°, and cosines are negative in the first and third quadrant.
So, you take 180° - 60° = 120° to get the angle in the first quadrant, and 180° + 60° = 240° to get the angle in the third quadrant.
Since only 120° is an option, the answer is B.
