If slope $\displaystyle{m}=\frac{{{y}_{{2}}-{y}_{{1}}}}{{{x}_{{2}}-{x}_{{1}}}}\usepackage{cmbright}\Huge\$ is undefined, then you must be dividing by zero. So $\displaystyle{x}_{{2}}-{x}_{{1}}={0}\usepackage{cmbright}\Huge\$ , or $\displaystyle{x}_{{2}}={x}_{{1}}\usepackage{cmbright}\Huge\$ . That is, all x-coordinates have the same value. All the points on a vertical line have the same x-coordinate.