Your equation can be factored to
... (cos +1)(2cos +1) = 0
The zero product rule tells you solutions will be
... cos = -1 . . . . . . angle = odd multiples of π
... cos = -1/2 . . . . angle = even multiples of π ± 2π/3
Together, these are {2π/3, π, 4π/3} +2kπ. You need to choose the solutions among these that are in your interval. [The interval is not given in your problem statement.]
