Gravitation question bank (Physics)

According to Kepler's second law of planetary motion, the magnitude of areal velocity $\left(\dfrac{\Delta A}{\Delta t}\right)$ of a planet of mass '$m$' revolving around the sun is constant and is equal to ('$p$' is the linear momentum and '$L$' is the angular momentum)

The mass of earth is 81 times the mass of the moon and the distance between their centres is $\mathrm{R}$. The distance from the centre of the earth where gravitational force will be zero is