The factor theorem says that if x=a is a zero then x–a is a factor. That applies whether a is real or complex. So if –9i is a zero then x+9i is a factor. If you want the coefficients to be real then the complex conjugate expression x–9i must also be a factor. Therefore is a factor. The other two zeros are x=2 and x=–2, so x–2 and x+2 are also factors and hence so is . Multiplying both pairs of factors together, you get (so your answer was close but not quite exact).

Try this one for yourself, along the same lines. If x = –4–2i is a zero, then x–(–4–2i) = x+4+2i is a factor ... .