A function fx y z whose level surfaces are equally spaced

Question

Question: 1. A function f(x, y, z) whose level surfaces are equally spaced planes perpendicular to the yz-plane.

2. A function f(x, y, z) whose level sets are concentric cylinders centered on the y-axis.

3. A nonlinear function f(x, y, z) whose level sets are parallel planes.