Moment of Inertia of square based pyramid?

[hewey13]

Homework Statement

A regular square pyramid (base length a, height b) is spun about its axis of symmetry z.
Calculate its moment of inertia about the z axis

Homework Equations

volume of pyramid
centre of mass

The Attempt at a Solution

have found volume and centre of mass. I know the answer to the moment of inertia but when i try to get to it I am always out by factors.

 

[vela]

Try looking at it as a stack of square slabs rotated about their center.