What is you "normal" method? If you are trying to factor them, you should give that up. You will not be able to do it. This is not necessarily obvious.
If f(x) = Your Quartic Expression
f(-1) > 0 and f(0) < 0 -- This means one of your roots is on (-1,0)
f(4) < 0 and f(5) > 0 -- This means another of your roots is on (4,5)
You must search for them. A simple bisection method is kind of fun.
There are no more Real roots, but if you drag out those two shown above, deflating f(x) to a new polynomial of lower degree, you will be left with a quadratic that can be solved easily for the two Complex Roots.
If g(x) = Your Cubic Expression
We know that you CAN solve this expression entirely. The process is painful. You probably do not want to see it, let alone do it.
By the same examination as above, we see:
g(-5) < 0
g(-4) > 0
There must be a Real Root on (-5,-4). Why?
g(-1) > 0
g(0) < 0
There must be a Real Root on (-1,0). Why?
g(2) < 0
g(3) > 0
There must be a Real Root on (2,3). Why?
In this case, you MAY wish to go hunting for all three Real roots, but this is more effort than is necessary. If you can find just one, you can reduce the problem to a mere quadratic and simply state the other two. Warning! You must find the first to wonderful precision of you expect the other two to be of much value. Deflated polynomials are VERY sensitive to bad precision.