Write the equation of the horizontal line that is tangent to the curve and is above the x-axis. Pause this video and see if you can have a go at it. So let's just make sure we're visualizing this right. Let me just draw a quick and dirty diagram. If that's my y-axis, this is my x-axis. I don't know exactly what that curve looks like, but imagine you have some type of a curve that looks something like this. Well, there would be two tangent lines that are horizontal based on how I've drawn it.