Let's say that f of x is equal to x times e to the negative 2x squared. And we want to find any critical numbers for f. So I encourage you to pause this video and think about, can you find any critical numbers of f? So assuming you've given a go at it, so let's just remind ourselves what a critical number is. So we would say c is a critical number of f if and only if, I'll write if with two f's, short for if and only if, f prime of c is equal to 0 or f prime of c is undefined. So if we look for the critical numbers for f, we want to figure out all the places where the derivative of this with respect to x is either equal to 0 or it is undefined. So let's think about how we can find the derivative of this. So let's see, f prime of x is going to be, well let's see, we're going to have to apply some combination of the product rule and the chain rule.