So I don't know if I completely understand this Identity. I am working on my homework and this identity is really bothering me and I can't figure it out:

Write the expression in terms of sine and cosine, and simplify so that no quotients appear in the final expression and all expressions are of only.

*So I know that the negative identity for cos is :

- so that will make the top become:

then using the pythagorian identity on top it becomes....

now is where I get lost.... the answer is supposed to be:

*I dont understand how I use the Negative-Angle Identity to make the bottom to be used with the Pythagorean Identities to get to the answer. Thank you in advance for your time and help.