a) (x + 1)(x + 2)(x + 3)(x + 4) - 24
= [(x + 1)(x + 4)].[(x + 2)(x + 3)] - 24
= (x2 + 5x + 4)(x2 + 5x + 6) - 24
= (x2 + 5x + 5 - 1)(x2 + 5x + 5 + 1) - 24
= (x2 + 5x + 5)2 - 1 - 24 = (x2 + 5x + 5)2 - 25
= (x2 + 5x)(x2 + 5x + 10)
= x(x + 5)(x2 + 5x + 10)