Solution
= Solution
{c}
The <multivariable chain rule> gives $u_s=2su_x+2tu_y$ and $u_t=-2tu_x+2su_y$. Therefore
$$
su_s-tu_t=2(s^2+t^2)u_x,
$$
so $\boxed{m=2}$. The $u_y$ terms cancel identically, independently of the particular <function> $u$.
= Solution
{c}
The <multivariable chain rule> gives $u_s=2su_x+2tu_y$ and $u_t=-2tu_x+2su_y$. Therefore
$$
su_s-tu_t=2(s^2+t^2)u_x,
$$
so $\boxed{m=2}$. The $u_y$ terms cancel identically, independently of the particular <function> $u$.