Substitute $\displaystyle{2}{w}\usepackage{cmbright}\Huge\$ for l in the second equation: $\displaystyle{2}{\left({2}{w}\right)}+{2}{w}={72}\usepackage{cmbright}\Huge\$

Simplify: $\displaystyle{4}{w}+{2}{w}={72}\usepackage{cmbright}\Huge\$ or $\displaystyle{6}{w}={72}\usepackage{cmbright}\Huge\$

Solve for w: $\displaystyle{w}={12}\usepackage{cmbright}\Huge\$

Substitute this value into the first equation: $\displaystyle{l}={2}{\left({12}\right)}={24}\usepackage{cmbright}\Huge\$