Formula for the area of a rectangle: $\displaystyle{A}={l}\cdot{w}\usepackage{cmbright}\Huge\$

So: $\displaystyle{300}={\left({w}+{5}\right)}{w}\usepackage{cmbright}\Huge\$

Subtract 300 from both sides: $\displaystyle{{w}}^{{2}}+{5}{w}-{300}={0}\usepackage{cmbright}\Huge\$

Factor: $\displaystyle{\left({w}+{20}\right)}{\left({w}-{15}\right)}={0}\usepackage{cmbright}\Huge\$

Use Zero Factor Property: $\displaystyle{w}+{20}={0}\usepackage{cmbright}\Huge\$ or $\displaystyle{w}-{15}={0}\usepackage{cmbright}\Huge\$

$\displaystyle{w}=-{20}\usepackage{cmbright}\Huge\$ or $\displaystyle{w}={15}\usepackage{cmbright}\Huge\$

The width cannot be negative, so discard $\displaystyle-{20}\usepackage{cmbright}\Huge\$ .

The width = 15 feet.

The length = $\displaystyle{w}+{5}={20}\usepackage{cmbright}\Huge\$ feet.