6*sin^2*x+5*cos(x)-7=0
6-6cos^2 x + 5cos x -7 = 0
6 cos^2 x - 5cos x +1=0
cos x =1/2 => x = +-pi/3+2pi*n,n принадлежит Z
cos x = 1/3 => x=+-arccos(1/3) +2pi*n