A thin$-$walled beam has the cross$-$section shown in the figure below. If the bearn is subjected to a bending moment $M_x$ in the plane of the vertical web:

If $I=418$ units ${?}^4,$Iyy$=231$ units ${?}^4,$ and Ixy$=-193$ units ${?}^4,$ find the direct stress distribution equation in the beam cross$-$section

$(10$ pts$)$