Answer
3 billion joules. Yes, the PE decreases by 3 billion J.
Work Step by Step
The work-energy theorem says that the net work done is the change in kinetic energy.
The KE change is 8 GJ - 5 GJ = 3 billion J.
The sum of KE and PE remains the same. KE increased, so potential energy decreased by the same amount, 3 billion J.
