It appears that you combined all three terms in the numerator, although they were not all like terms. The first step is:

$\displaystyle\frac{{{6}{x}-{12}}}{{{x}-{3}}}-\frac{{{2}{x}}}{{{x}-{3}}}=\frac{{{6}{x}-{12}-{2}{x}}}{{{x}-{3}}}\usepackage{cmbright}\Huge\$