8 sin^2 x - 12 cos x = - 7

8(1-cos^2x)-12cosx+7=0
8-8cos^2x-12cosx+7=0
-8a^2-12a+15=0
8a^2+12a-15=0
D=144-4*8(-15)=624
a1,2=-12+-sqrt(624)/16