Chapter 9 - Section 9.3 - Three-Dimensional Coordinate Geometry - 9.3 Exercises - Page 652: 10

The plane $y=-1$ shown

So we are given the equation $y=-1$. In 2D this would be a line but we are thinking in a 3D context. Let's think, all values of x and z would be correct as long a y is -1. This would be a plane extending in all directions. In fact this plane is parallel to the one in number 8.