1. The problem statement, all variables and given/known data Please find question attached. The context is a lecture course on seismic theory for exploration geophysics. 2. Relevant equations The substitution for the elasticity tensor made in my solution is given in the lecture notes. Please find my attempt at the solution attached (hopefully readable). For one, I have no idea how to get a negative term in my solution. I wonder if it could be to do with the direction of motion (the sound waves travelling down into the Earth) or perhaps an identity I'm missing. Thanks in advance for help.