How can I diagonalize this symmetric matrix?

[Denisse]

\begin{array}{ccc}
2 & 1 & 0 \\
1 & 2 & 0 \\
0 & 0 & 3 \end{array}

 

[rock.freak667]

If A is your matrix, then first you will need to get your eigenvalues λ by solving

| A-λI| = 0 where I is the identity matrix.

Then you will need to get your eigenvectors.