Help with getting an expression for Electric field from a variable Voltage

quackzilla

It gives V(x,y,x) = A(x^2,-3y^2,z^2) and asks for E.


What I have done:

E is equal to V/d , and I assume d = (x,y,x)

Dividing them I get E= A(x,-3y,z)

What am I doing wrong?

I also have another question:

In every plane parallel to the xz-plane the equipotential contours are circles. What is the radius of the equipotential contour corresponding to V=1280V and y=2m? (this is an extension of the previous question).

I have a test tomorrow, and there are a few things I still don't know how to do. Any hints on how to do these would greatly help my studying.