You may have incorrectly distributed the 4 and gotten $\displaystyle{8}{x}-{1}\usepackage{cmbright}\Huge\$ , instead of $\displaystyle{8}{x}-{4}\usepackage{cmbright}\Huge\$ , and then incorrectly computed $\displaystyle{8}\cdot{5}-{1}\usepackage{cmbright}\Huge\$ by subtracting first, instead of multiplying.