Solve trigonometric function: cos 2x = sin 3x * sin x

cos 2x = sin 3x * sin x
using formula
sin α * sin β = 1/2 (cos(α − β) − cos(α + β))
I'm finding hard time to figure out anything useful after making it into:
cos( 2x) = 1/2 (cos (3x-x) - cos(3x+x))
2(cos(2x)) = - cos(2x)
Am I even starting it right?