2 cos^2x + 5 cos x = 3

2 cos² x + 5 cos x = 3
2 cos² x + 5 cos x - 3 = 0
(2 cos x - 1)(cos x + 3) = 0

2 cos x = 1
cos x = 1/2
x = 60°
x = 300°

nilai x
60° dan 300°