First observe that a cubic has at least one real root. So if all the rootsOriginally Posted by unicorn
lie on then should be a real root.
This checks out, so we know that is a factor of the cubic,
and so we use long division to find the remaining quadratic factor:
Now we can use that quadratic formula to verify that the two remaining
roots (which are the roots of ) have real parts
(which they do).