The moment of inertia of the ring about an axis perpendicular to the plane of ring is Ir=mk2.
The moment of inertia of the sphere is
Is=52mR2s
It is given that
mk2=mR2r=52mRs2...(i)
From equation (i)
Rr=52Rs
From the question, it can be written that
RsRr=x2
⇒RsRr=52⇒x=5