Chapter P - P.2 - Linear Models and Rates of Change - Exercises - Page 16: 14

The points are $(-4,0),(-4,1),(-4,2)$.

When $m$ is undefined the given line is vertical and all of the points on it just have the same $x$ coordinate so we can choose any three points with $x$ coordinate that equals $-4$. For example: $(-4,0),(-4,1),(-4,2)$.