Chapter 7 Edexcel further trip q C3

Hiya, could someone please help me on this q:

Use the double angle formulae and the identity cos(A+B) = cosAcosB - sinAsinB to obtain the expression for cos3x in terms of powers of cos x only.

So far I've got that A = 2x and B=x (or vice versa) but could somebody please help me on the following steps?

Any help appreciated.
Many thanks,
tat

Now expand cos(a+b) and play around with it to see if you can get everything in terms of powers of cos.

Thanks. I'm supposed to express the sin 2x and cos 2x in terms of a single x first, right?

Exactly you can do this. You are correct, you only want single x'a in there.