A thin plate $10$ mm thick is subjected to a twisting moment of $5$ Nm$/$mm along each edge in addition to the bending moments of Mx $=10$ Nm$/$mm and My $=5$ Nm$/$mm$.$ Determine the principal moments in the plate, the planes on which they act, and the corresponding principal stresses. Please show me how to arrive at answers to Planes and Principal stresses The answers given are $(-31\mathrm{.}7$ deg, $+58\mathrm{.}3$deg; $+-786$ N$/$mm$^2$; $114$ N$/$mm$^2$