You may have correctly gotten the equation $\displaystyle{100}={l}{\left({l}-{15}\right)}\usepackage{cmbright}\Huge\$ and mistakenly thought you could set $\displaystyle{l}-{15}={100}\usepackage{cmbright}\Huge\$ . To solve a quadratic equation you must first get all the terms on one side. Then factor and use the Zero Product Property. Always check your answers by going back to the original problem.