Mmm,
Mat's first method is for equations on degree 2
Factorization method can be used if you can work out the factors.
You can use remainder theorem / factor theroem to help find them.
https://www.khanacademy.org/math/algebra2/polynomial_and_rational/polynomial-remainder-theorem-tutorial/v/polynomial-remainder-theorem
You can use graphing with an online graphing program such as Desmos.
you can use an online calculator such as our web2calc one or Wolfram|Alpha
If you give an exact example then we can better direct you :)