A nonrotating spherical planet with no atmosphere has mass M and radius R. A projectile of mass m is launched radially from the surface of the planet with initial speed . The potential energy of the projectile-planet system, as a function of the projectile’s distance r from the center of the planet, is given by . The greatest distance from the center of the planet that the projectile reaches is A) infinity
B) R
C) 7/5 R
D) 4/3 R
E) R

I know the answer is D, 4/3 R, but I'm not sure how to get to that answer.