You may have subtracted the coefficients and the radicands. Rewrite each radical using $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{a}}\cdot\sqrt{{b}}\usepackage{cmbright}\Huge\$ :

$\displaystyle{8}\sqrt{{{72}}}-{4}\sqrt{{{50}}}={8}\sqrt{{{36}\cdot{2}}}-{4}\sqrt{{{25}\cdot{2}}}={8}\sqrt{{36}}\cdot\sqrt{{2}}-{4}\sqrt{{25}}\cdot\sqrt{{2}}\usepackage{cmbright}\Huge\$

Simplify and then combine like terms.