We can apply Perpendicular axis theorem here on x axis & y axis and get I ’, moment of inertia in z axis .
Iz = Ix + Iy , now as because of symmetry Ix & Iy are same so Iz = I ’ = 2I = Mr 2 / 2
Now we can apply parallel axis theorem to find I ’’. I ’’ = I ’ + MR 2 = 3 / 2 ( MR 2 )