You can use the rational root theorem to find the first root since it must be a factor of -6. Since 1 is always easy to check start there: . Hence r = 1 is a root.
By the factor theorem, if 1 is a root then (r-1) must be a factor of the cubic
where A, B and C are constants to be found either by comparing coefficients or by long division.
Once you've found the quadratic remember to check if it factors (it obviously does judging by the answer)