While solving the equation $\displaystyle{100}={{l}}^{{2}}-{15}{l}\usepackage{cmbright}\Huge\$ , you may have incorrectly rewritten $\displaystyle-{15}{l}\usepackage{cmbright}\Huge\$ as $\displaystyle-{25}{l}+{4}{l}\usepackage{cmbright}\Huge\$ , resulting in a length of 25.