Answer:

Note that the approximate value you should calculate properly with the inverse cosine function of your calculator to the number of digits you want.
Explanation:
you already have a cosine. You might as well change that sine in a cosine too:

(note that in the second line I also switched every sign by multiplying by -1 just to have a positive lead coefficient - just a pet peeve of mine. Now replace
and let's solve

At this point we're back to the replacement. We have two options
