One model for a certain planet has a core of radius R and mass M surrounded by

Question:

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. It M = 4.1 x I024 kg and R = 6.0 x 106 m, what is the gravitational acceleration of a particle at points
(a) R and
(b) 3R from the center of the planet?