If $LaTeX: \displaystyle f(x)=- \frac{7}{3 - 6 x}$ and $LaTeX: \displaystyle g(x)=7 x^{3} + x^{2} - 3 x + 9$ find $LaTeX: \displaystyle (f\circ g)(x)$ .

Evaluating $LaTeX: \displaystyle f(x)$ at $LaTeX: \displaystyle g(x)$ gives $LaTeX: \displaystyle (f\circ g)(x)=f(g(x))=- \frac{7}{- 42 x^{3} - 6 x^{2} + 18 x - 51}$ .