So we would want to take the limit as x approaches a here. Either way, we're doing a very similar, we're doing the exact same thing. We're finding, we're taking, we have an expression for the slope of a secant line, and then we're bringing those x values of those points closer and closer together, closer and closer together, so the slopes of those secant lines better and better and better approximate that slope of the tangent line, and at the limit, it does become the slope of the tangent line. That is the derivative of, or that's the definition of the derivative. So this is the kind of the more standard definition of a derivative. It would give you your derivative as a function of x, and then you can then input your x that you want, the particular value of x, or you could use the alternate form of the derivative. If you know that, hey, look, I'm just looking to find the derivative exactly at a, I need a general function of f, then you could do this, but they're doing the same thing.