In Ireland and Rosen's Number theory book, ch.8, p94, it explains how to use Jacobi sums to find the number of solutions to the equation $x^{3}+y^{3}=1$.
From the book: $N(x^{3}+y^{3}=1)=p-\chi(-1)-\chi^{2}(-1)+2ReJ(\chi,\chi)=p-2+2ReJ(\chi,\chi)$.
My question is, how do we get the part $2ReJ(\chi,\chi)$? I would like a detailed explanation, although I think it must be easy since it was omitted. thanks