given that z = 8i express z in polar form and obtain the three cube roots of 8i in polar form.
You should be able to see that $\displaystyle z = 8 \text{cis} \left( \frac{\pi}{2} + 2n\pi\right) $ where n is an integer (did you plot z on an Argand diagram?). Now use deMoivre's Theorem.