velocity potential

Uniform flow with velocity U makes an angle $\theta$ with the y-axis, as shown in the figure The velocity potential ($\phi$), is

For the function $$\phi = a{x^2}y - {y^3}$$ to represent the velocity potential of an ideal fluid, $${\nabla ^2}\,\,\phi $$ should be equal to zero. In that case, the value of $$'a...