Add 18 to both sides of the equation: $\displaystyle{{x}}^{{2}}-{6}{x}+{18}={0}\usepackage{cmbright}\Huge\$

Substitute $\displaystyle{a}={1},{b}=-{6}\usepackage{cmbright}\Huge\$ , and $\displaystyle{c}={18}\usepackage{cmbright}\Huge\$ into the Quadratic Formula:

$\displaystyle{x}=\frac{{-{b}\pm\sqrt{{{{b}}^{{2}}-{4}{a}{c}}}}}{{{2}{a}}}\usepackage{cmbright}\Huge\$

$\displaystyle{x}=\frac{{-{\left(-{6}\right)}\pm\sqrt{{{{\left(-{6}\right)}}^{{2}}-{4}{\left({1}\right)}{\left({18}\right)}}}}}{{{2}\cdot{1}}}=\frac{{{6}\pm\sqrt{{{36}-{72}}}}}{{2}}=\frac{{{6}\pm\sqrt{{-{36}}}}}{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle=\frac{{{6}\pm\sqrt{{36}}{i}}}{{2}}=\frac{{{6}\pm{6}{i}}}{{2}}=\frac{{6}}{{2}}\pm\frac{{{6}}}{{2}}{i}={3}\pm{3}{i}\usepackage{cmbright}\Huge\$