It's not y explicitly defined as a function of x. For any x value, you actually have two possible y's that satisfy this relationship right over here. So you might be tempted to maybe split this up into two separate functions of x. You could say y is equal to the positive square root of one minus x squared. And you could say y is equal to the negative square root of one minus x squared. Take the derivatives of each of these separately, and you would be able to find the derivative for any x, or the derivative of the slope of the tangent line at any point. But what I wanna do in this video is literally leverage the chain rule to take the derivative implicitly so that I don't have to explicitly define y as a function of x either way.