This line contains the points (10, 0) and (10, 1). Its slope would be:

$\displaystyle\frac{{{y}_{{2}}-{y}_{{1}}}}{{{x}_{{2}}-{x}_{{1}}}}=\frac{{{1}-{0}}}{{{10}-{10}}}=\frac{{1}}{{0}}\usepackage{cmbright}\Huge\$

This is undefined.