Split or Irreducible

Let E be a subset of the Complex numbers and assume r=e^(2*pi*i)/p be
in E where p is a prime number. Let a be arbitrary in E. Show that
the polynomial f(x)=(x^p)-a is irreducible or splits in E[x].

I know that r must be the key to the problem but I don't know how to
use it.

James

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