4 cos^2 x - 12 sin x +3=0
4(1 - sin^2 x ) - 12 sin x + 3 = 0
4 - 4 sin ^2 - 12 sin x + 3 = 0
7 - 4 sin^2 - 12 sin x = 0
sin x = t
7 - 4 t^2- 12 t = 0
4 t^ 2 -12 t +7= 0
t1= -7/2
t2=1/2
sinx = -7/2. не имеет решений
sinx= 1/2
x = π/6+2kπ, k є z
x = 5π/6 +2kπ, k є z
