One model for a certain planet has a core of radius R and mass M surrounded by an outer shell of inner radius R, outer radius 2R, and mass 4M. If M = 7.89 x 1024 kg and R = 2.69 x 10 m, what is the gravitational acceleration of a particle at points (a) R and (b) 3R from the center of the planet? (a) Number Units > (b) Number Units eTextbook and Media Hint Save for Later Attempts: 0 of 1 used Submit