`D = (x - 2)^3 - (x - 3)(x^2 + 3x + 9) + 6(x + 1)^2`
`D = x^3 - 6x^2 + 12x - 8 - (x^3 + 3x^2 + 9x - 3x^2 - 9x - 27) + 6(x^2 + 2x + 1)`
`D = x^3 - 6x^2 + 12x - 8 - x^3 - 3x^2 - 9x + 3x^2 + 9x + 27 + 6x^2 + 12x + 6`
`D = (x^3 - x^3) + (6x^2 - 6x^2 + 3x^2 - 3x^2) + (9x - 9x + 12x + 12x) + (-8 + 6 + 27)`
`D = 24x + 25`