First of all, you'll need a 2, somewhere.

|z| = cis(x/3) really doesn't make quite enough sense. It is as if you ALMOST know where you are going.

Start with

Just apply the theorem from DeMoivre.

for k = 0, 1, 2

k = 0 leads to

k = 1 leads to

k = 2 leads to

One for free. You show us the next one.