You probably correctly rewrote the equation as $\displaystyle{{x}}^{{2}}-{2}{x}-{15}={0}\usepackage{cmbright}\Huge\$ and used the Quadratic Formula, $\displaystyle{x}=\frac{{-{b}\pm\sqrt{{{{b}}^{{2}}-{4}{a}{c}}}}}{{{2}{a}}}\usepackage{cmbright}\Huge\$ , but made a mistake when finding $\displaystyle-{b}\usepackage{cmbright}\Huge\$ .