Вот держи
3sin2x - 8 sin^2x =7
3*2sincosx - 8 sin^2x - 7 sin^2x - 7 cos^2x = 0
6sinxcosx - 15sin^2x - 7cos^2x = 0 /:(-1)
15sin^2x - 6sinxcosx + 7cos^2x = 0 /:cos^2x≠0
15tg^2x - 6tgx + 7 = 0
tgx = t
15t^2 - 6t + 7 = 0
D = 36 - 4*15*7 < 0
нет решений. !!!!!