So the derivative of c of our cost with respect to x is going to be equal to 40 times x minus 180 times x to the negative 2 power. Now this seems, well, it's defined for all x except for x equaling 0. But x equaling 0 is not interesting to us as a critical point because then we're going to have a degenerate. This is going to have no base at all. So we don't want to worry about that critical point. We would have no volume at all. So it would not work out.