Here is one way by completing the cube:
And there is a formula in terms of the coefficients.
But as I have been saying to evene and corrupted, it is much easier to do numerically.
Due to teacher complaints about cheating and copyright violation we are restricted in how much and what type help we can give for homework questions.
Do you have some work of your own that I can improve or help on? If you show some work, some effort, then you can be helped. It is as simple as that.