A 56.5-kg skateboarder starts out with a speed of 1.87 m/s. He does 116 J of work on himself by pushing with his feet against the ground. In addition, friction does -201 J of work on him. In both cases, the forces doing the work are non-conservative. The final speed of the skateboarder is 9.40 m/s. Calculate the change (PEf - PE0) in the gravitational potential energy.