Given:

$\displaystyle (-1) \over (6)$ = $\displaystyle \cos x$

with 90 < x < 180.

Find:

$\displaystyle \cos (x) $ = $\displaystyle ( -1 ) \over (2) $ $\displaystyle X$

So I actually did this:

$\displaystyle \cos^2 (X)$ $\displaystyle = $ $\displaystyle (1) + (-1/6) \over 2 $

$\displaystyle (6) - (1) \over 12 $

And simplified to:

$\displaystyle 1 \over 2 $ $\displaystyle \sqrt 15 $

But this is not an answer choice. I feel like I have missed a step. Thank you.