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

.

Calculate the principal stresses, given that each principal stress,

satisifies det( )=0

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

http://img28.imageshack.us/img28/844/diagram1e.png

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