Subtract 10 from both sides: $\displaystyle{{\left({x}-{10}\right)}}^{{2}}={90}\usepackage{cmbright}\Huge\$

Use the Square Root Property: $\displaystyle{x}-{10}=\pm\sqrt{{90}}\usepackage{cmbright}\Huge\$

Add 10 to both sides: $\displaystyle{x}={10}\pm\sqrt{{90}}\usepackage{cmbright}\Huge\$

Simplify the radical: $\displaystyle{x}={10}\pm\sqrt{{{9}\cdot{10}}}={10}\pm{3}\sqrt{{10}}\usepackage{cmbright}\Huge\$