x^3-xy^2
=x(x^2-y^2)
=x(x+y)(x-y)
ax^2-a
=a(x^2-1)
=a(x-1)(x+1)
2x^2-8
=2(x^2-4)
=2(x+2)(x-2)
2a^3-8a
=2a(a^2-4)
=2a(a-2)(a+2)