$(2x^2 -4)^2 + 9 \\ = (2x^2+5-9)^2 + 9 \\ = (2x^2+5)^2 - 18(2x^2+5) + 81 + 9 \\ = (2x^2+5)^2 - 36x^2 \\ = (2x^2-6x+5)(2x^2+6x+5)$