For x to the third equaling zero, well that's only gonna happen when x is equal to zero. And four minus five x equaling zero, we'll add five x to both sides. You get four is equal to five x. Divide both sides by five, you get four fifths is equal to x. So here, these are the two places where our derivative is equal to zero. Now are there any places where our derivative is undefined? Well, our function right over here is just a straight up polynomial. Our derivative is another polynomial.