(a^2b^2+ab)^2-(ab+4)^2 =(a^2b^2+ab+ab+4)(a^2b^2+ab-ab-4) =[(ab)^2+2ab+4][(ab)^2-2^2] =[(ab)^2+2ab+4](ab-2)(ab+2) =[(ab)^3-2^3](ab+2) =(a^3b^3-8)(ab+2)