1. The problem statement, all variables and given/known data 2. Relevant equations Mechanical conservation 3. The attempt at a solution It says the energy at height d = (1/2)mv^2 + 2mg(L-d) Isn't it just mgL = mgd + (1/2)mv^2 ? Since mgd is the final potential energy, at height d above the reference point which is taken to be the lowest point.