Answer:
-x^2 + 18x - 9.
Step-by-step explanation:
(x + 3)^2 - 2(x - 3)^2
= x^2 + 6x + 9 - 2(x^2 - 6x + 9)
= x^2 + 6x + 9 - 2x^2 + 12x - 18
= -x^2 + 18x - 9.
