$\displaystyle\frac{{{3}{{x}}^{{3}}-{3}{{x}}^{{2}}-{36}{x}}}{{{{x}}^{{2}}-{x}-{12}}}=\frac{{{3}{x}{\left({x}+{3}\right)}{\left({x}-{4}\right)}}}{{{\left({x}+{3}\right)}{\left({x}-{4}\right)}}}\usepackage{cmbright}\Huge\$ , so the domain of the expression excludes $\displaystyle{x}=-{3}\usepackage{cmbright}\Huge\$ and 4. Simplifying the expression, you find that $\displaystyle\frac{{{3}{x}{\left({x}+{3}\right)}{\left({x}-{4}\right)}}}{{{\left({x}+{3}\right)}{\left({x}-{4}\right)}}}={3}{x}\usepackage{cmbright}\Huge\$ .