The terms "cubic" and "linear" refer to the form of a polynomial that is a sum of monomials. A monomial is a coefficient multiplied by some power of x. Therefore, to decide if this polynomial is cubic or linear, you have to multiply through and to represent this polynomial as a sum of powers of x with some coefficients.

Note, however, that whether it is cubic or linear depends only on the highest power of any monomial, so you don't need to find the coefficients.