Verify (13.73). Assume that $k^2 \ll M_Z^2$ and that the target contains equal numbers of $u$ and $d$ quarks (i.e., it is an isoscalar target), and neglect antiquarks. Show that, in the standard model,
$$
\begin{aligned}
& a_1=-\frac{3}{4}\left(1-\frac{20}{9} \sin ^2 \theta_W\right), \\
& a_2=-\frac{3}{4}\left(1-4 \sin ^2 \theta_W\right) .
\end{aligned}
$$