Aerial Density of the lamina = M / ab , M = mass of lamina, a and b are dimensions of the lamina.
Let us take an axis passing through the central line of rectangle and perpendicular to the base "a".
Take a strip of width dx and length b. Its mass = dm = b *dx *M/ab = Mdx/a
Moment of Inertial = I =
If moment of inertia is found about the central axis along the other dimension,
I = M b²/12