Ok, here is how I tried to solve and where I stucked at
I don't know what to do next
Are you expected to get exact solutions by hand? In which case you have a difficult job ahead because I don't think there are rational roots. You will need to follow the process explained here: The "Cubic Formula"
Or are decimal approximations sufficient? In which case, use technology to get solutions to the required accuracy.
Posting the whole question, including the context it's taken from, will shed light on this if you cannot answer these questions.