The answer given is wrong.
In order to correctly find the solution the trig functions must all be expanded out and combined.
This results in a 4th degree polynomial in either sin(x) or cos(x) depending on how you expand things.
I was working on writing up the LaTex for the solution when the editor hiccupped on me and I lost it all.
See if you can reproduce it. It's not difficult. A couple of trig identities and some algebra.