We divide the thin rectangular sheet in small parts of height b and length dr. All these sheets are parallel to b. The infinitesimal moment of inertia of one of these small parts is
where
Now we find the moment of inertia by integrating from
to 
The moment of inertia is
(from (-a/2) to
(a/2))
where
Now we find the moment of inertia by integrating from
The moment of inertia is