cosY= -sqrt3/2
and
cosY= -sqrt3/2 for y in [pi,2pi]
The question ask to solve these equations
Keep in mind that cosine is negative in the second and third quadrants.
The second question is asking for particular angles within the interval $\displaystyle \pi\leq y\leq2\pi$. Note that you will only have one solution.
keep in mind that $\displaystyle \cos\left(\tfrac{\pi}{6}\right)=\tfrac{\sqrt{3}}{2 }$ Change the angle to get $\displaystyle \cos(?)=-\tfrac{\sqrt{3}}{2}$
The first question wants you to generalize the solution. This can be a little tricky. Find several solutions to the equation, and then try to find a pattern given in the coefficients of the angles. Try this one and let us know where you get stuck...if you get stuck...
I hope this helps.
--Chris