- b +/- sqrt( b^2 - 4ac)
x = ---------------------------
2a
6 cos² x + 5 cos x - 4 , a = 6, b = 5 , c = - 4
- 5 +/- sqrt( 5^2 - 46( - 4)) - 5 +/- sqrt(121)
x = ----------------------------------- = --------------------------
2*6 12
sqrt(121) = 11
x =( -5 + 11)/12 = 1/2
x = ( - 5 - 11)/12 = - 16/12 = - 4/3
( cosx - 1/2)(cosx + 4/3) = 0
cosx - 1/2 = 0 ⇒ cosx = 1/2 ⇒ x = 60 because cos 60 = 1/2
cosx + 4/3 = 0 ⇒ cosx = - 4/3 = - 1.333 ⇒ it is not a solution