The remainder theorem says that if 
is a factor of a polynomial 
, then the remainder upon dividing 
is 0 so that there exists a lower degree polynomial 
as the quotient:
Using the fact that
is a factor, you can find a quadratic which is easy to factorize further.
Synthetic division yields
which can be factored further as
So,
The roots are then
.
Using the fact that
Synthetic division yields
which can be factored further as
So,
The roots are then