First, you probably wrote the slope as $\displaystyle\frac{{{r}{u}{n}}}{{{r}{i}{s}{e}}}\usepackage{cmbright}\Huge\$ instead of $\displaystyle\frac{{{r}{i}{s}{e}}}{{{r}{u}{n}}}\usepackage{cmbright}\Huge\$ . Second, you forgot that when you move two units down, you are moving in a negative direction, so the rise is $\displaystyle-{2}\usepackage{cmbright}\Huge\$ .