You probably substituted 6 for b and $\displaystyle-{18}\usepackage{cmbright}\Huge\$ for c in the Quadratic Formula. The first step is to add 18 to both sides of the equation: $\displaystyle{{x}}^{{2}}-{6}{x}+{18}={0}\usepackage{cmbright}\Huge\$

Now substitute $\displaystyle{a}={1},{b}=-{6}\usepackage{cmbright}\Huge\$ , and $\displaystyle{c}={18}\usepackage{cmbright}\Huge\$ into $\displaystyle{x}=\frac{{-{b}\pm\sqrt{{{{b}}^{{2}}-{4}{a}{c}}}}}{{{2}{a}}}\usepackage{cmbright}\Huge\$ .