What is the magnitude of potential energy at infinity distance?

The gravitational field due to a mass distribution is $E = K/x^3$ in the $X$-direction ($K$ is a constant). Taking the gravitational potential to be zero at infinity,its value at a distance $x$ is:

Consider two configurations of a system of three particles of masses $m, 2m$ and $3m$. The work done by an external agent in changing the configuration of the system from figure $(i)$ to figure $(ii)$ is:

Four particles each of mass $m$ are placed at four vertices of a rectangle having side lengths $3 l_o$ and $4 l_o$. The magnitude of the gravitational potential energy of the system in units of $\frac{Gm^2}{l_o}$ is:

Two heavy spheres each of mass $100 \; kg$ and radius $0.10 \; m$ are placed $1.0 \; m$ apart on a horizontal table. What is the gravitational force and potential at the mid-point of the line joining the centres of the spheres? Is an object placed at that point in equilibrium? If so,is the equilibrium stable or unstable?

$A$ point particle is held on the axis of a ring of mass $m$ and radius $r$ at a distance $r$ from its centre $C$. When released,it reaches $C$ under the gravitational attraction of the ring. Its speed at $C$ will be