(1)
3x^6-3x^2
=3x^2(x^4-1)
=3x^2(x^2+1)(x^2-1)
=3x^2(x^2+1)(x+1)(x-1)
(2)
x^5-2x^3-8x
=x(x^4-2x^2-8)
=x(x^2-4)(x^2+2)
=x(x^2+2)(x+2)(x-2)