We actually want to optimize, or actually maybe a better way to talk about it, we want to minimize this expression right over here. We want to find the m and the b values that minimize it. And to help visualize it, we're going to start breaking into a little bit of three-dimensional calculus here. But hopefully it won't be too daunting. If you've done any partial derivatives it won't be difficult. This is a surface. If you view that you have the x and y data points, everything here is a constant except for the m's and the b's.