The derivative of this with respect to x would just be f of x. In fact, the whole reason why we introduce this variable t here is this thing right over here is actually a function of x, because x is this upper bound. And it would've been weird if we had x as an upper bound, or at least confusing, and we're also integrating with respect to x. So we just have to pick kind of another placeholder variable that doesn't have to be t. It could be alpha, it could be gamma, it could be a, b, or c, whatever we choose. But this is still right over here. This is a function of x. But when you take the derivative of both sides, you realize that the function f, which is graphed here, and if this were the x-axis, then this would be f of x.