Let's express the potential energy of the sphere at the top of the incline.
The PE of the sphere at the top is the product of the mass of the sphere m, the acceleration due to gravity g, and the height of the incline htop.
PEtop = mghtop
We can solve for htop
htop = PEtop / mg
= 8 J / (0.5kg)(9.81m/s2)
= 1.63 m
The height of the incline htop is 1.63m.
The PE of the sphere halfway down the incline is such that the sphere is at a height htop/2
PE1/2 = mg(htop/2)
= 3.99 J
≈ 4 J
It makes sense that the sphere's gravitational potential energy would be half its original value when the sphere is halfway down the incline.
I hope this helped.