First simplify each radical. Then, if the radicands and the indices are the same, you have like terms. Combine them just as you would combine $\displaystyle{2}{a}-{a}\usepackage{cmbright}\Huge\$ .

$\displaystyle\sqrt{{{20}}}-\sqrt{{{5}}}=\sqrt{{{4}\cdot{5}}}-\sqrt{{{5}}}=\sqrt{{4}}\cdot\sqrt{{5}}-\sqrt{{5}}={2}\sqrt{{{5}}}-{1}\sqrt{{{5}}}=\sqrt{{{5}}}\usepackage{cmbright}\Huge\$