The denominator is $\displaystyle\sqrt{{{21}}}\usepackage{cmbright}\Huge\$ , so multiply the fraction by $\displaystyle\frac{\sqrt{{{21}}}}{\sqrt{{{21}}}}\usepackage{cmbright}\Huge\$

$\displaystyle{\left(\frac{{7}}{\sqrt{{{21}}}}\right)}\cdot\frac{{\sqrt{{{21}}}}}{{\sqrt{{{21}}}}}=\frac{{{7}\sqrt{{{21}}}}}{\sqrt{{{441}}}}=\frac{{{7}\sqrt{{{21}}}}}{{21}}=\frac{\sqrt{{{21}}}}{{3}}\usepackage{cmbright}\Huge\$