(a^2+b^2-9)^2-4a^2b^2
=(a^2+b^2-9-2ab)(a^2+b^2-9+2ab)
=(a^2-2ab+b^2-9)(a^2+2ab+b^2-9)
=((a-b)^2-9)((a+b)^2-9)
=(a-b+3)(a-b-3)(a+b+3)(a+b-3)