So let's write down a differential equation. The derivative of y with respect to x is equal to four y over x. And what we'll see in this video is the solution to a differential equation isn't a value or a set of values. It's a function or a set of functions. But before we go about actually trying to solve this or figure out all of the solutions, let's test whether certain equations, certain functions are solutions to this differential equation. So for example, if I have y is equal to four x, is this a solution to this differential equation? Pause the video and see if you can figure it out.