Hello, I am having troubles with a question involving trigonometric identities. I have always had trouble with these types of problems, so I need as much help as possible.

To the problem:

"Prove that the following is an identity: "

Here is my attempt:

R.H.S

I don't know what to do from here. On the left hand start I can't see where to even start. Any help at all is very much appreciated. I really do not understand these problems.

Thank you.