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

To find the critical numbers solve $LaTeX: \displaystyle f'(x) = 0$ . The derivative is $LaTeX: \displaystyle f'(x) = 21 x^{2} - 8 x - 5$ . Solving $LaTeX: \displaystyle 21 x^{2} - 8 x - 5 = 0$ gives $LaTeX: \displaystyle x = \left[ - \frac{1}{3}, \ \frac{5}{7}\right]$ . Using the 2nd derivative test gives:
$LaTeX: \displaystyle f''\left( - \frac{1}{3} \right) = -22$ which is less than zero, so the function is concave down and $LaTeX: \displaystyle f\left(- \frac{1}{3}\right) = - \frac{55}{27}$ is a local maximum.
$LaTeX: \displaystyle f''\left( \frac{5}{7} \right) = 22$ which is greater than zero, so the function is concave up and $LaTeX: \displaystyle f\left(\frac{5}{7}\right) = - \frac{297}{49}$ is a local minimum.