Let's simplify the expression step by step:

Expand the first set of parentheses:

X(x+2)(x-2) = X(x^2 - 2x + 2x - 4) = X(x^2 - 4)

Expand the second set of parentheses:

(x-3)(x^2 + 3x + 9) = x(x^2 + 3x + 9) - 3(x^2 + 3x + 9)
= x^3 + 3x^2 + 9x - 3x^2 - 9x - 27
= x^3 - 27

Subtract the second expression from the first one:

X(x^2 - 4) - (x^3 - 27)
= Xx^2 - 4X - x^3 + 27
= - x^3 + Xx^2 - 4X + 27

Therefore, the simplified expression is - x^3 + Xx^2 - 4X + 27.