Using the notation
$$
N_p^\pi(z) \equiv \frac{1}{\sigma} \frac{d \sigma}{d z}(\mathrm{ep} \rightarrow \pi \mathrm{X}) .
$$
show that, in the valence quark approximation for $\mathrm{p}, \mathrm{n}$,
$$
\frac{\int d z\left[N_n^{\pi^{+}}-N_n^{\pi^{-}}\right]}{\int d z\left[N_p^{\pi^{+}}-N_p^{\pi^{-}}\right]}=\frac{2}{7} .
$$