Because of the conjugate property, the function will also have the zero -3 + 2i
So....let the function be f(x) and we have that
f(x) = a( x + 1)( x - ( - 3 - 2i) ) ( x - (-3 + 2i) ) simplify
f(x) = a ( x + 1) ( x^2 -x (-3 + 2i) -x(-3 -2i) + ( -3 + 2i) (-3 - 2i) )
f(x) = a ( x + 1) ( x^2 + 3x - 2xi + 3x + 2xi + 9 - 4i^2 )
f(x) = a ( x + 1) (x^2 + 6x + 9 + 4)
f(x) = a (x + 1)(x^2 + 6x + 13)
f(x) = a ( x^3 + 6x^2 + 13x + x^2 + 6x + 13 )
f(x) = a ( x^3 + 7x^2 + 19x + 13)
And since f(-9) = -6, we can find "a" as
-6 = a ( (-9)^3 + 7(-9)^2 + 19(-9) + 13)
-6 = a ( -320)
a = 6 / 320 = 3/160
So.....f(x) = (3/160) ( x^3 + 7x^2 + 19x + 13)