Fill in the steps leading to (7.130).
This yields $p, \alpha=0$ and $p=P\left(y_3\right)$, with $P^{\prime}\left(y_3\right) / k=\operatorname{divt}$. Then, $P^{\prime \prime}\left(y_3\right)$ vanishes and $(7.130)$ reduces to
$$
C=k d i v \mathbf{t}
$$