Could you help me with this physics question Consider a plan

Could you help me with this physics question?

Consider a planet moving in the potential of the form U(r) = -alpha/r + beta/r^2. Find an angular shift deltapi in its trajectory between two consecutive revolutions around the center. Under which condition the trajectory of this plane is a closed one?

V(r) = -a/r -b/r^2
m*ar = mr\" = Fr = -dV/dr = a/r^2 + 2b/r^3
So, trajectory T(r) = a/r^2 + 2b/r^3