设二元函数 $z=z(x, y)$ 有二阶连续偏导数, 且满足
$$
6 \frac{\partial^2 z}{\partial x^2}+\frac{\partial^2 z}{\partial x \partial y}-\frac{\partial^2 z}{\partial y^2}=1,
$$
令变量 $\left\{\begin{array}{l}u=x-2 y \\ v=x+3 y\end{array}\right.$, 那么 $\frac{\partial^2 z}{\partial u \partial v}=$