Basically it's two things: 1. answering the question "how do we get the next x" for a function x=y. If x=y, then if you increase x, you also increase y, so dx

Basically it's two things: 1. answering the question "how do we get the next x" for a function x=y. If x=y, then if you increase x, you also increase y, so dx/dy=1. Figuring out dx/dy is fun when dealing with things like 3x^2+5x+7=y. But take the game of Pong, for example. A simple Pong game has a ball and the direction of the ball can be controlled by two variables - horizonal velocity (H) and vertical velocity (V). Each frame, you take the ball's X and Y and add H and V to it, to move the ball. When the ball collides with something, just multiply by -1 (to flip the sign) to reverse the ball. If you divide V by something like 0.0001 each frame, you will implement gravity. Want the ball to have a gravity or other "pull" that results in it hitting a point in a specific number of frames? Well ... someone who knew what they were doing would know what to do. That's all I got. 2. If you have a few points for x=y, you should be able to figure out dx/dy somehow. I think that's called integrals.