How would you going about finding the plane of reflection of the matrix:

$\displaystyle

\frac{1}{3}

(\begin{array}{ccc}

2 & 2 & -1 \\

2 & -1 & 2 \\

-1& 2 & 2 \end{array})]

$

Not looking for an answer, just looking for a method of finding the plane. Thanks!