Linear algebra question

Let A be a n x n symmetric matrix, and let L be an eigenvalue of A with corresponding eigenvector X.

Show that L is also an eigenvalue of the matrix P(transposed)AP where P is an orthogonal matrix. State the corresponding eigenvector of P(transposed)AP.

How is the result modified if P(transposed)AP is a diagonal matrix D?

I have no idea even where to start. Thanks guys.