Factor 54 so that you have a perfect square times a number with no square factors:

$\displaystyle\sqrt{{54}}=\sqrt{{{9}\cdot{6}}}=\sqrt{{{3}\cdot{3}\cdot{6}}}=\sqrt{{{{3}}^{{2}}\cdot{6}}}\usepackage{cmbright}\Huge\$

Use $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{a}}\cdot\sqrt{{b}}\usepackage{cmbright}\Huge\$ to rewrite the expression: $\displaystyle\sqrt{{{{3}}^{{2}}\cdot{6}}}=\sqrt{{{{3}}^{{2}}}}\cdot\sqrt{{6}}\usepackage{cmbright}\Huge\$

Take the positive square root of $\displaystyle{{3}}^{{2}}\usepackage{cmbright}\Huge\$ : $\displaystyle{3}\sqrt{{6}}\usepackage{cmbright}\Huge\$