X^6-x^3-6 is the question.

Whenever you have exponents that are multiples of each other consider using substitution as follows:

Let u = x^3, then u^2=x^6, and your expression becomes: u^2-u-6. The quadratic should be straightfoward to factor(though if you need help feel free to ask), and after you factor it you can substiute x back in.

Hope this helps.