Always look at the previous part as its most likely used.
Rearrange part (a) as i notice how they look pretty similar and i'm going to let
Compare this to (b) and you notice how they're pretty similar. If you let

and multiply everything by 2, you end up with the same equation as in (b).
So now you solve

for theta, and you end up with one solution being 40 degrees, so one root being

as
