Find the difference quotient of $LaTeX: \displaystyle f(x)=- 5 x^{3} + 7 x^{2} - 8 x - 9$ .

The difference quotient is $LaTeX: \displaystyle \frac{f(x+h)-f(x)}{h}$ . Evaluating $LaTeX: \displaystyle f(x+h)=- 8 h - 8 x - 5 \left(h + x\right)^{3} + 7 \left(h + x\right)^{2} - 9$ and expanding gives $LaTeX: \displaystyle f(x+h)=- 5 h^{3} - 15 h^{2} x + 7 h^{2} - 15 h x^{2} + 14 h x - 8 h - 5 x^{3} + 7 x^{2} - 8 x - 9$ Evaluating the difference quotient gives $LaTeX: \displaystyle \frac{(- 5 h^{3} - 15 h^{2} x + 7 h^{2} - 15 h x^{2} + 14 h x - 8 h - 5 x^{3} + 7 x^{2} - 8 x - 9)-(- 5 x^{3} + 7 x^{2} - 8 x - 9)}{h}$ Simplifying gives $LaTeX: \displaystyle \frac{- 5 h^{3} - 15 h^{2} x + 7 h^{2} - 15 h x^{2} + 14 h x - 8 h}{h}=- 5 h^{2} - 15 h x + 7 h - 15 x^{2} + 14 x - 8$