Correct. Add $\displaystyle{{\left(\frac{{-{20}}}{{2}}\right)}}^{{2}}={{\left(-{10}\right)}}^{{2}}={100}\usepackage{cmbright}\Huge\$ to both sides to complete the square: $\displaystyle{{x}}^{{2}}-{20}{x}+{100}={0}\usepackage{cmbright}\Huge\$

Write the left side as the square of a binomial: $\displaystyle{{\left({x}-{10}\right)}}^{{2}}={0}\usepackage{cmbright}\Huge\$

Take the square root of both sides: $\displaystyle{x}-{10}={0}\usepackage{cmbright}\Huge\$

Solve for x: $\displaystyle{x}={10}\usepackage{cmbright}\Huge\$