$2-9$ Stress components at a point are known, except for the component ${}_{yy},$

$S=\left[\begin{array}{ccc}1& 0& 1\\ 0& {}_{yy}& 2\\ 1& 2& 0\end{array}\right]$

Select ${}_{yy}$ so that there will be a traction$-$free plane through the point. Determine its unit normal.

After ${}_{yy}$ is found above, try to determine $(1)$ The mean stress $($or average stress$)$ and pressure, $(2)$ the von Mises stress $($or equivalent stress$),$ and $(3)$ the maximum shear stress.