Critical Numbers

Find and classify the critical points of the following function

$f(x) = x^3 - 3x^2 - 9x + 2$

Find the critical points of the following function

$f(x) = 6x^5 + 33x^4 - 30x^3 + 100$

Find the critical numbers of the function

$f(x) = x^2 - 4x$

Find the critical numbers for the following function $f(x) = x^{\frac{2}{3}}$