Considering four identical discs as 1,2,3 and 4.

Moment of inertia of disc about diagonal is I1=I3=4MR2
Now using parallel axis theorem,
Moment of inertia of disc 2 and 4 is
I2=I4=4MR2+MR2=45MR2
So, net moment of inertia about OO′ is
I=I1+I2+I3+I4
=2(4MR2)+2(45MR2)
=2MR2+25MR2=3MR2
Putting R=2a
I=43Ma2, so,x=3