Solution
= Solution
Here $v_x=-3x^2+3y^2+4y$ and $v_y=6xy+4x$. The <harmonic conjugate> and resulting polynomial are
$$
\boxed{v=-x^3+3xy^2+4xy+C,\qquad f(z)=2z^2-iz^3+iC,\quad C\in\mathbb R.}
$$
Indeed $\operatorname{Re}(-iz^3)=3x^2y-y^3$ and $\operatorname{Re}(2z^2)=2x^2-2y^2$.