Since there is a hole in the larger mass, we can calculate the attractive force between the two objects if there were no hole and then subtract off the attractive force between the smaller mass and a mass the size of the hole the same distance away.
To find the mass given the density, we need to multiply the density times the volume of the sphere.
Now that we know the attractive force that the spheres would have if there weren't a hole, we see how much attractive force the hole is responsible for.
Now we can just subtract off the force that the hole is responsible for from the solid sphere
