Figure shows a uniform solid block of mass M and edge lengths a, b and c. Its moment of inertia about an axis through one edge and perpendicular (as shown) to the large face of the block is : 

\(\frac{M}{3} (a^2 + b^2)\)

(Moment of Inertia)_CG : I

\(I_{CG} = M\frac{a^ + b^2}{12}\)

According to the theorem of parallel axis :

(Moment of Inertia)_{required axis} : I

\(I = I_{CG} + M(OA)^2\)

  = \(M(\frac{a^2 + b^2}{12}) + M(\frac{a^2 + b^2}{4})\)

  = \(M(\frac{a^2 + b^2}{3})\)