The potential of a certain electrostatic field has the form = a (x2 + Y2) + bz2,

Question:

The potential of a certain electrostatic field has the form φ = a (x2 + Y2) + bz2, where a and b are constants. Find the magnitude and direction of the electric field strength vector. What shape has the equipotential surfaces in the following cases?
(a) a > 0, b > 0;
(b) a > 0, b <0?