problem: Expand $((2x^2+3x+3)-(x^2+6x-6))(x+3)$.
solution: We have that
  \begin{align*} &((2x^2+3x+3)-(x^2+6x-6))(x+3)\\ &\qquad= (x^2-3x+9)(x+3) \\ &\qquad= x(x^2-3x+9) + 3(x^2-3x+9) \\ &\qquad= x^3 -3x^2 +9x +3x^2 -9x +27 \\ &\qquad= \boxed{x^3+27}. \end{align*}