Solution
= Solution
{c}
In <Cartesian coordinates>, write the constant <vector> as $\mathbf c=(c_1,c_2,c_3)$. The <product rule> and constancy of its components give
$$
\nabla\cdot(\phi\mathbf c)
=\partial_x(c_1\phi)+\partial_y(c_2\phi)+\partial_z(c_3\phi)
=c_1\phi_x+c_2\phi_y+c_3\phi_z.
$$
The last expression is the <dot product> with the <gradient>, so
$$
\boxed{\nabla\cdot(\phi\mathbf c)=\mathbf c\cdot\nabla\phi.}
$$