Two solid spherical planets of equal radii R having masses 4M and 9M. Their centre are separated by a distance 6R. A projectile of mass m is sent from the planet of mass 4 M towards the heavier planet. What is the distance r of the projectile from the centre of the heavier planet where the gravitational force on the projectile is zero ?

1)1.4R

2) 1.8R

3) 1.5R

4) 3.6R