If $z = x - iy$ and ${z^{{1 \over 3}}} = p + iq$, then ${{\left( {{x \over p} + {y \over q}} \right)} \over {\left( {{p^2} + {q^2}} \right)}}$ is equal to :

Question

If $z = x - iy$ and ${z^{{1 \over 3}}} = p + iq$, then

${{\left( {{x \over p} + {y \over q}} \right)} \over {\left( {{p^2} + {q^2}} \right)}}$ is equal to :