**Equation: **

**In the picture, the x-axis ranges from -10 to 10, the
y-axis ranges from -20 to 20, and the z-axis ranges from -30 to 30.
Traces in planes parallel to the xy-plane are parabolas; traces in planes
parallel to the xz-plane or the yz-plane are lines (for positive y-values
and for all x-values; for negative y-values, a plane parallel to the xz-plane
does not intersect the surface).**