Solution (source code)

= Solution
{c}

For the <vector field> $\mathbf u=(-y,x,0)$, the Cartesian formula for the <curl> gives
$$
\nabla\times\mathbf u
=(\partial_y0-\partial_zx,\partial_z(-y)-\partial_x0,
\partial_xx-\partial_y(-y))
=\boxed{(0,0,2)=2\mathbf e_3}.
$$