In Mathematica you could say
In[66]:= Graphics[{PointSize[Large],
Point[{Re[#], Im[#]} & /@ ComplexExpand[x /. Solve[x^3 == -I, x]]]},
Frame -> True, Axes -> True, AxesLabel -> {"Re", "Im"}]
to see the roots in the Gaussian plane. And you could say this to see their polar form
In[67]:= {Abs[#], Arg[#]} & /@ ComplexExpand[x /. Solve[x^3 == -I, x]]
Out[67]= {{1, \[Pi]/2}, {1, -((5 \[Pi])/6)}, {1, -(\[Pi]/6)}}
their length is 1, of course and they trisect the unit circle, as usual.
