In general, if n is even, $\displaystyle{\sqrt[{{n}}]{{{{x}}^{{n}}}}}={\left|{x}\right|}\usepackage{cmbright}\Huge\$ . When $\displaystyle{n}={2}\usepackage{cmbright}\Huge\$ , this says that $\displaystyle\sqrt{{{{x}}^{{2}}}}={\left|{x}\right|}\usepackage{cmbright}\Huge\$ , so the simplified form of $\displaystyle\sqrt{{{36}{{x}}^{{2}}}}={6}{\left|{x}\right|}\usepackage{cmbright}\Huge\$ .