Answer:
I had no idea how to solve this on edge either. I ended up looking on you tube and found this guy named Larry. He did it step by step in a short video!!! I will put it down below so you can watch it if you need help.
Explanation:
The answer is B.√2-√6 over 4
video name on you tube:
cos 7pi/12 find the exact value of the trig function