So we want to figure out the points on that curve where the tangent line is vertical. So let's just remind ourselves what the slope of a tangent line is, or what it isn't, I guess. This may be a better way to think about it. If you have a horizontal line, so let me draw a horizontal line. If you have a horizontal line like that, well then your slope is zero, or you could say your rate of change of y with respect to x is equal to zero. But what about a vertical line? If you have a vertical line like that, what is your rate of change of y with respect to x?