The question is: A 15.7 kg block is dragged over a horizontal surface by a 68.7N force acting 17 degrees to the horizontal. The block is displaced 4.65m, and the kinetic friction coefficient is 0.3. What is the energy lost to friction? My thinking is that it would be equal to the negative work done by the frictional force (E taken out of the system), but the computer does not recognize this as correct. What am I missing?