I think you need to be more specific as to where you are at.

You should have

By deMoivre's theorem:

You do not have an infinite range, because you are given the conditions that

So you want to pick the values k for which the angle lies between and

These values of k are: (Note: it's 3, not -3 because your range includes , but not )

To put it into exponential form as required, you'll need euler's formula: