You may have found $\displaystyle\frac{{{4}-{0}}}{{{0}-{4}}}\usepackage{cmbright}\Huge\$ instead of $\displaystyle\frac{{{4}-{0}}}{{{4}-{0}}}\usepackage{cmbright}\Huge\$ or $\displaystyle\frac{{{0}-{4}}}{{{0}-{4}}}\usepackage{cmbright}\Huge\$ when you were calculating the slope.