And if we're going to put an x as one of our bounds of integration, well, we'll use a different variable for our integration here. So let me write it this way. So if I had the integral from a to x of f prime of u du, well, what is this going to be equal to? Well, this is going to be, by the same logic, the antiderivative of this evaluated at x, so f of x minus f of a. Minus f of a. Or if we wanted to solve for f of x, we could add f of a to both sides, and we would get f of x is equal to the integral from a to x of f prime of u du, and once again, why did I pick u? Well, I just needed some letter other than x, since I already used x as one of my bounds of integration.