## Calculus: Early Transcendentals 8th Edition

The equation $y=-2$ represents a vertical plane parallel to the xz-plane that is shifted 2 units in the negative y direction.
The equation sets a value for y that is independent of x or z. If $f(x,z)=y$, then $f(x,z)$ will equal -2 at every point on the xz-plane.