You may have 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\$ , and also used $\displaystyle{c}=-{4}\usepackage{cmbright}\Huge\$ . You should be using $\displaystyle{a}={1},{b}=-{6}\usepackage{cmbright}\Huge\$ , and $\displaystyle{c}={4}\usepackage{cmbright}\Huge\$ .