can someone help me with this problem?

Find https://instruct.math.lsa.umich.edu/...34fb482cf1.png as a function of https://instruct.math.lsa.umich.edu/...dd0b8b8e91.png if https://instruct.math.lsa.umich.edu/...29a82d6161.png.

i know it should be easy but i must be doing something stupid incorrectly because the answer i got was wrong(Headbang)