Well, it's defined everywhere else, so let's think about where the derivative is equal to zero. So when does this thing equal zero? So when does 2x minus 512 x to the negative 3 equal zero? Well, we can add 512 x to the negative 3 to both sides, so you get 2x is equal to 512 x to the negative 3rd power. We can multiply both sides times x to the 3rd power, multiply both sides times x to the 3rd, so all of the x's go away on the right-hand side. So you get 2x to the 4th is equal to 512. We can divide both sides by 2, and you get x to the 4th power is equal to 256.