Hello, I am struggling with this problem. It is probably the easiest problem ever... What I did: The plane has 2 stress components. σn and σs. σn is a multiple of (l, m, k) vector. For σs, I made up a vector (a, b, c) which is orthogonal to (l, m, k). And I equated all vectors. I'm probably doing something wrong. Any help is appreciated!