tan(x).sin(x) = 3sin(x) - sec(x)
sinx/cosx . sinx = 3sinx - 1/cosx
sin^2 x /cosx = 3sinx - 1/cosx
sin^2 x = 3sinx cos x - 1
sin^2 x - 3 sinx cosx + 1 = 0
sin^2 x - 3 sinx cos x + cos^2 x + sin^2 x = 0
2sin^2 x - 3sinx cosx + cos^2 x = 0
(2sin x - cosx) (sinx - cos x) = 0
continue on yourself from here.