You may have set up the correct equation, $\displaystyle{2}{w}+{2}{\left({w}+{8}\right)}={76}\usepackage{cmbright}\Huge\$ , where w is the width, but then forgot to use the distributive property. The left side of the equation is equal to $\displaystyle{2}{w}+{2}{w}+{16}\usepackage{cmbright}\Huge\$ , not $\displaystyle{2}{w}+{2}{w}+{8}\usepackage{cmbright}\Huge\$ .