So the slope of the tangent line is equal to zero. And the derivative exists in a neighborhood around c, and most of the functions we deal with, if it's differentiable at c, it tends to be differentiable in the neighborhood around c. And then we also assume that the second derivative exists, it's twice-differentiable. Well then, we might be dealing with a maximum point, we might be dealing with a minimum point, or we might not know what we're dealing with. And it might be neither a minimum or a maximum point. But using the second derivative test, if we take the second derivative, and if we see that the second derivative is indeed less than zero, then we have a relative maximum point. So this is a situation that we started with right up there. If our second derivative is greater than zero, then we are in this situation right here, we're concave upwards.