Trigonometry question......help!

Question

Which of the following third-quadrant angles satisfies the equation 2cos²x-cosx=1

a)0

b)2π/3

c)7π/6

d)4π/3

Answer 1

2 cos² x - cos x - 1 = 0

(2cos x + 1)(cos x - 1) = 0

2cos x = -1

or

cos x = 1

Can you solve for x and answer?