vague question?
do you mean (x-4)*x*(x+4) or (x-4)*(x+4)?
(x-4)*x*(x+4) = (x-4)*(x*(x+4)) = (x-4)*(x*x + x*4) = (x-4)*(x^2+4x) = (x*x^2+x*4x) - (4*x^2+4*4x) = x^3 +4x^2 -4x^2 - 16x = x^3 - 16x ?
(x-4)*(x+4) = x*(x+4) - 4*(x+4) = x*x +x*4 - 4*x - 4*4 = x^2 -16 ?
