pls help me ive been stuck for a long time

Let  $f(a) = a^3+2a^2-9a-18$

⇒ (a + 2) is a factor of f(a) 

So dividing f(a) by (a+2) we get 

$f(a) = (a+2)(a^2-9)$

         $=(a+2)(a+3)(a-3)$

∴ Factors = -2, 3, -3