a)  x( x + 2)( x^2 - 1) - 24
= x(x + 2)(x - 1)(x + 1) - 24
= [x(x + 1)]. [(x + 2)(x - 1)] - 24
= (x^2 + x). (x^2 + x - 2) - 24
= (x^2 + x)^2 - 2(x^2 + x) - 24
= (x^2 + x)^2 - 6(x^2 + x) + 4(x^2 + x) - 24
=(x^2 + x) (x^2 + x - 6) + 4.(x^2 + x - 6)
= (x^2 + x + 4)(x^2 + x - 6 )