And we want to raise it to the 1 third power. We get 1.65. So it's approximately equal to 1.65 as our critical point. Now the way the problem is asked, we're only getting one legitimate critical point here. So that's probably going to be the x at which we achieve a minimum value. But let's use our second derivative test just in case to make sure that we're definitely concave upwards over here, in which case that this will definitely be the x value at which we achieve a minimum value. So the second derivative of our cost function is just the derivative of this, which is going to be equal to 40 minus 180 times negative 2, which is negative 360.