Plus another 30 is going to be 180. So it's going to be plus 180 times x times x to the negative 2, 180x to the negative 1 power. So we finally have cost as a function of x. Now we're ready to optimize. To optimize, we just have to figure out what are the critical points here and whether those critical points are minimum or maximum value. So let's see what we can do. So to find a critical point, we take the derivative, figure out where the derivative is undefined equal to 0.