So our cost as a function of x is going to be 20x squared. 36 times 5. Let's see, 30 times 5 is 150. 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.