Because the two radicals are the same, these are like terms, and you can combine them just as you would combine $\displaystyle{9}{a}+{2}{a}\usepackage{cmbright}\Huge\$ .

$\displaystyle{9}\sqrt{{{5}}}+{2}\sqrt{{{5}}}={\left({9}+{2}\right)}\sqrt{{5}}={11}\sqrt{{{5}}}\usepackage{cmbright}\Huge\$