Incorrect. First rewrite each radical using $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{a}}\cdot\sqrt{{b}}\usepackage{cmbright}\Huge\$ :

$\displaystyle\sqrt{{{300}{{x}}^{{2}}{{y}}^{{2}}}}-{3}\sqrt{{{27}{{x}}^{{2}}{{y}}^{{2}}}}=\sqrt{{{100}\cdot{3}{{x}}^{{2}}{{y}}^{{2}}}}-{3}\sqrt{{{9}\cdot{3}{{x}}^{{2}}{{y}}^{{2}}}}=\sqrt{{100}}\sqrt{{3}}\sqrt{{{{x}}^{{2}}}}\sqrt{{{{y}}^{{2}}}}-{3}\sqrt{{9}}\sqrt{{{3}}}\sqrt{{{{x}}^{{2}}}}\sqrt{{{{y}}^{{2}}}}\usepackage{cmbright}\Huge\$

Simplify and then combine like terms. The correct answer is 3.