This line contains the points $\displaystyle{\left({0},-{10}\right)}\usepackage{cmbright}\Huge\$ and $\displaystyle{\left({1},-{10}\right)}\usepackage{cmbright}\Huge\$ . Its slope would be:

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

The slope is 0.