Don't know if this is the right section. Sincerely hope so.

I'm having trouble with this problem (I'll try to translate it right...).

The edge of a cube measures x/2 - 1, in cm. Determine the rate of change in volume for x = 10.

I tried to find the derivative of [(x/2 - 1)^3] and then the result of x = 10, but I didn't get 24 cm3 per 1 cm, which is the right answer. I'ld like to know what I'm doing wrong.