Both x and 5 should be multiplied by $\displaystyle{2}{x}\usepackage{cmbright}\Huge\$ . You did multiply 5 by $\displaystyle{2}{x}\usepackage{cmbright}\Huge\$ . However, your $\displaystyle{{x}}^{{2}}\usepackage{cmbright}\Huge\$ term shows that you multiplied x by itself, not by $\displaystyle{2}{x}\usepackage{cmbright}\Huge\$ .