Moment of inertia

find the moment of inertia of a uniform square plate of mass "m" and edge "a"...about one of its diagonals?

This is to be done by the symmetry

see the moment of inertia about the one diagonal is equal to it about the other diagonal.

Hence their sum will give the moment of inertia about the Z axis.

This hint will suffice?

http://targetiit.com/iit_jee_forum/posts/moment_of_inertia_3348.html

The link above gives a even more general solution

see if you are able to understand this one :)

otherwise I will try to explain the current question again :)