I can't figure it out please help!

Post what you think the answer is so we can critique it. Site rules are that we won't do homework, won't give you the answer. So show your work, what you think it is, even if you think it's stupid, and we can help. We can't give you the answer though, that's something you have to figure out.

Are you familiar with the identies for cos(2x)? Start with that and it comes right out.