If you start with $\displaystyle{{x}}^{{2}}+{b}{x}\usepackage{cmbright}\Huge\$ , add the number $\displaystyle{{\left(\frac{{b}}{{2}}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ to complete the square.

$\displaystyle{b}=-{12}\usepackage{cmbright}\Huge\$ , so add: $\displaystyle{{\left(\frac{{b}}{{2}}\right)}}^{{2}}={{\left(\frac{{-{12}}}{{2}}\right)}}^{{2}}={{\left(-{6}\right)}}^{{2}}={36}\usepackage{cmbright}\Huge\$

The resulting trinomial is: $\displaystyle{{x}}^{{2}}-{12}{x}+{36}={\left({x}-{6}\right)}{\left({x}-{6}\right)}={{\left({x}-{6}\right)}}^{{2}}\usepackage{cmbright}\Huge\$