(x+1)^3-(x-1)^3-6(x+1)^2=x^3+3x^2+3x+1-x^3+3x^2-3x+1-6(x^2+2x+1)
=6x^2+2-6x^2-12x-6
=-12x-4
(x+1)3-(x-1)3-6(x+1)2
=(x+1)2.[(x+1)+6.1]-(x-1)3
=(x+1)2.(x+1+6)-(x-1)3
=(x+1)2.(x+7)-(x-1)3
= ( x3+3x2+3x+1) - ( x3-3x2+3x-1) - 6(x2+2x+1) = x3+3x2+3x+1 - x3+3x2-3x+1 - 6x2-12x-6
= -12x - 4