Check the Stokes' theorem for the function \vec v = y\hat z
Here it is over a tetrahedron.
Stokes' theorem suggests:
\int_s {(\nabla\times \vec v).d\vec a = \oint_p\vec v.d\vec r
For the right hand side I computed the line integral from (a,0,0)--->(0,2a,0)--->(0,0,a)--->(a,0,0);
which...