His answer was (x- +2) (x^2-9)
a) His mistake was he has x^2 in one of the parenthesis, to be fully factored you cannot have an exponent as part of the answer.
b) x^3 - 9x + 2x^2 -18
Reorder the terms:
x^3 +2x^2 -9x -18
Group the first two terms and the last two terms
(x^3 + 2x^2) -9x-18
Factor out the greatest common factor from each group:
x^2(x+2) - 9(x+2)
Factor the polynomial by factoring out the common factor once x+2:
(x+2) (x^2-9)
Rewrite 9 as 3^2:
(x+2) (x^2-3^2)
Factor one last time:
(x+2) (x+3) (x-3)
