If you really have "no idea" about this then you better read/check your books/class notes. Nobody not having an idea of this should be trying this kind of problems.
It is my opinion that "fairly lengthy explanations" belong in the classroom or with a particular teacher. Here you can usually expect a little explanation and
some hints and/or complete resolution but without much explanation.
In this case it's hard to decide what you're supposed to know and thus how to approach this problem: have you studied complex exponential
Fourier series? If that's the case, and since it seems to be that
, then we get that
Check the above and then evaluate (trivial integral)