Find the derivative of f(x) = (1/6)x²(x-x²) in two ways.
a) f'(x) = (1/2)x² - x³
b) f'(x) = (1/2)x² - x³ + (1/6)x⁴
c) f'(x) = (1/3)x³ - (1/2)x²
d) f'(x) = (1/3)x³ - x²