You may have subtracted the coefficients and the radicands, obtaining $\displaystyle{2}\sqrt{{{63}}}\usepackage{cmbright}\Huge\$ , and then simplified. Rewrite each radical using $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{a}}\cdot\sqrt{{b}}\usepackage{cmbright}\Huge\$ :

$\displaystyle{6}\sqrt{{{75}}}-{4}\sqrt{{{12}}}={6}\sqrt{{{25}\cdot{3}}}-{4}\sqrt{{{4}\cdot{3}}}={6}\sqrt{{25}}\cdot\sqrt{{3}}-{4}\sqrt{{4}}\cdot\sqrt{{3}}\usepackage{cmbright}\Huge\$

Simplify and then combine like terms.