1. The problem statement, all variables and given/known data In the third picture , I dont understand the circled part , add up the values in the diagonal .. How to do that ? I dont understand how to get k13 , k14 , k21 , k22 , k33, k41 and k42 . 2. Relevant equations 3. The attempt at a solution As we see in the second picture , the k21 and k22 = -20 and 20 respectively , why in the third picture , the k1 and k22 become -20 and 40 ? Is the solution provided wrong ?