You only added one length and one width of the frame together. To find the perimeter of the frame, you need to add both lengths and both widths of the frame: $\displaystyle{\left({2}{a}+{6}\right)}+{\left({a}+{3}\right)}+{\left({2}{a}+{6}\right)}+{\left({a}+{3}\right)}\usepackage{cmbright}\Huge\$ .