1) to find the characteristic polynomial you just do the subtract x from all the diagonal entries and find the determinant. I got (x+4)(x-5)(x+3)
2) The eigenvalues are the roots to the characteristic polynomial.
3) you just gotta solve for each of the eigenvalues. They are distinct, so you will get solutions to all of these, the are the corresponding eigenvectors for the eigenvalues.
4) Your P is going to just be the collection of these eigenvectors, so the matrix with ith column .
I don't know if you have talked about Jordan Canonical Form, but as soon as you see it has 3 distinct eigenvalues you can be sure it is diagonalizable, and this is your matrix that you use to get it into its diagonal form. It should equal the matrix with diagonal entries -4, 5, -3, the eigenvalues. And in this order. This is unique up to permutation of the diagonal elements. You should check all these calculations for understanding at each step as I did them kinda fast, but this should be a good start.