(I really hope this is in the correct place.)

The equation is

I need to find the four roots but I only managed three, and one was by looking and guessing. In an exam, I won't have time to start guessing so I was wondering if there was a more methodic, secure way to solve this.

I guessed (using the calculator) that 3 was one root. Through that, I guessed -3. Then I divided the equation by

and got

which, in turn, gave me

and

. But that's what I don't understand, I divided the original equation by

, how am I still getting that as a result? And since the highest power is 4, I assumed that I would find four roots but I've only found three.

Also, once I have all four answers, the question stated that I need to write them out in order, from the most amount of times that the root appears in the equation. I don't exactly understand what this means. I would post the original question, but it's not in English and therefore wouldn't really be of any value. I translated as best as I could.

Thank you.