The moment of inertia of hollow sphere (mass M) of inner radius R and outer radius 2R having material of uniform density about a diametric axis is---

I tried to divide the problem into two parts the moment of inertia of the sphere of radius 2R($I_1$) and the moment of inertia of smaller sphere of radius R($I_2$) Then moment of body=$I_1-I_2$

But I am not getting the correct answer

Here is the solution I found but couldn't getting how they used $I=1/2 M R^2$.it is not the moment of inertia of a sphere.