When combining the like terms $\displaystyle{3}{a}+{3}{a}+{a}+{a}\usepackage{cmbright}\Huge\$ , you may have forgotten that $\displaystyle{a}\usepackage{cmbright}\Huge\$ has a coefficient of 1 and only added $\displaystyle{3}{a}+{3}{a}\usepackage{cmbright}\Huge\$ .