4(3x^2-x-1)(x^2+2x-3)-(4x^2+x-4)^2
=4(3x^2-x-1)(x^2+2x-3)-[(3x^2-x-1)+(x^2+2x-3)]2^
=4(3x^2-x-1)(x^2+2x-3)-(3x^2-x-1)^2-2(3x^2-x-1)(x^2+2x-3)-(x^2+2x-3)^2
=-(3x^2-x-1)^2+2(3x^2-x-1)(x^2+2x-3)-(x^2+2x-3)^2
=-[(3x^2-x-1)-(x^2+2x-3)]^2
=-(2x^2-3x+2)^2