b) Aire $=12 x^{3}-18 x$

Step 1 ：Find the first derivative of the function: \(f'(x) = 36x^2 - 18\)

Step 2 ：Set the first derivative equal to zero: \(36x^2 - 18 = 0\)

Step 3 ：Solve for x: \(x = \pm\frac{\sqrt{2}}{2}\)

Step 4 ：\(\boxed{\text{The critical points are } x = -\frac{\sqrt{2}}{2} \text{ and } x = \frac{\sqrt{2}}{2}}\)