Are you sure you can use only the difference formula? Most people derive it with the sum formula using cos(2x) = cos(x+x)
So Im supposed to use the difference formula to figure out cos(2x). This is what I have so far:
cos(2x)=cos(3x-x)
cos(3x-x)=(cos3x)(cosx)+(sin3x)(sinx)
Im just not sure how to finish solving! (my math homework is done online so it's supposed to be as simplified as possible).