(5 x^4 - 9 x^3 - 14 x^2) + (12 x^2 + 4x - 8) =x^2 (5 x^2 - 9x - 14) +
+ 4(3x^2 + x - 2) = x^2*5(x+1)(x - 14/5) + 4*3(x + 1)(x - 2/3)=
=x^2(x+1)(5x - 14) + 4(x+1)(3x - 2)=
=(x+1)( x^2(5x - 14) +4( 3x- 2)) = (x+1)(5x^3 - 14 x^2 + 12x - 8)=
=(x+1)(5x^3 - 14x^2 + 8 x + 4x - 8)=(x+1)( (5x^3 - 14x^2 +8x)+(4x-8))=
=(x+1)(x(5x^2 -14x+8) +4(x-2))=(x+1)(x(x-2)(5x-4) + 4(x-2))=
=(x+1)(x-2)(5x^2 - 4x +4)= (x+1)(x-2)(5x^2 - 4x +4) =
= - (x+1)(x-2)( - 5x^2 + 4x - 4).