This value for the width could be the result of solving $\displaystyle{300}={5}{w}\usepackage{cmbright}\Huge\$ . You may have correctly found the quadratic equation that needs to be solved, but then omitted the $\displaystyle{{w}}^{{2}}\usepackage{cmbright}\Huge\$ term. You should check your answer by verifying that the product of the length and width is 300.