Help on physics problem?

Slash has completed his TOE-theory of everything! he can now create his own universe! he decides to try different forms of gravitational force between two bodies. what form would Kepler's third law take if it were an inverse cube law? suppose slash wanted all the planets to have the same speeds in their orbits. what form would newtons law of gravity take? what form would it take if slash wanted all the planets to have the same period? assume circular orbits and that the force is still proportional to the product of the masses.