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.