Hi

I am having trouble with the following problem:

1) An element of a lamina is subjected to normal and tangential stresses on each of its four sides. The associated stress matrix is

$\displaystyle \begin{bmatrix}

12 & -8 \\ -8 & 15

\end{bmatrix}$.

Calculate the principal stresses, given that each principal stress, $\displaystyle \lambda$

satisifies det($\displaystyle A-\lambda I$)=0

The question also had this diagram which i don't understand what is suppose to represent?

I started finding the det for A, however what should i do next?

det(A) = 12*15- -8*-8

det(A) = 180-64

det(A) = 116

P.S