If w is the width, then $\displaystyle{w}+{8}\usepackage{cmbright}\Huge\$ is the length. The perimeter is twice the width plus twice the length. So $\displaystyle{2}{w}+{2}{\left({w}+{8}\right)}={76}\usepackage{cmbright}\Huge\$ .

Using the distributive property: $\displaystyle{2}{w}+{2}{w}+{16}={76}\usepackage{cmbright}\Huge\$

Add like terms and subtract 16 from both sides: $\displaystyle{4}{w}={60}\usepackage{cmbright}\Huge\$

Divide both sides by 4: $\displaystyle{w}={15}\usepackage{cmbright}\Huge\$ , so the length is $\displaystyle{15}+{8}={23}\usepackage{cmbright}\Huge\$ feet.