Calculus 8th Edition

$x=\frac{1}{3}$
Find the critical numbers of $f(x) = 4+ \frac{1}{3}x - \frac{1}{2}x^2$ Differentiate and set the derivative $= 0$ to find the critical numbers. $f'(x) = \frac{1}{3} - x$ $0=\frac{1}{3} - x$ $x=\frac{1}{3}$