Find the local maximum and minimum of $LaTeX: \displaystyle f(x) = - 7 x^{3} - 13 x^{2} - 5 x - 4$ .

To find the critical numbers solve $LaTeX: \displaystyle f'(x) = 0$ . The derivative is $LaTeX: \displaystyle f'(x) = - 21 x^{2} - 26 x - 5$ . Solving $LaTeX: \displaystyle - 21 x^{2} - 26 x - 5 = 0$ gives $LaTeX: \displaystyle x = \left[ -1, \ - \frac{5}{21}\right]$ . Using the 2nd derivative test gives:
$LaTeX: \displaystyle f''\left( -1 \right) = 16$ which is greater than zero, so the function is concave up and $LaTeX: \displaystyle f\left(-1\right) = -5$ is a local minimum.
$LaTeX: \displaystyle f''\left( - \frac{5}{21} \right) = -16$ which is less than zero, so the function is concave down and $LaTeX: \displaystyle f\left(- \frac{5}{21}\right) = - \frac{4567}{1323}$ is a local maximum.