You incorrectly simplifed $\displaystyle{2}{a}+{6}\usepackage{cmbright}\Huge\$ to $\displaystyle{8}{a}\usepackage{cmbright}\Huge\$ and $\displaystyle{a}+{3}\usepackage{cmbright}\Huge\$ to $\displaystyle{4}{a}\usepackage{cmbright}\Huge\$ , and then added the lengths and widths, but you cannot combine unlike terms. The perimeter is the sum of the four sides: $\displaystyle{\left({2}{a}+{6}\right)}+{\left({a}+{3}\right)}+{\left({2}{a}+{6}\right)}+{\left({a}+{3}\right)}\usepackage{cmbright}\Huge\$ . Regroup and combine like terms.