Volumes - Linear Relationships

Hi

For this question:

A symmetrical pier of height 5 m has an elliptical base with equation x^2/25 + y^2/4 = 1 and slopes to a parallel elliptical top with equation x^2/9 + y^2 = 1. If the cross sections of the area parallel to the base are also elliptical find the volume of the pier given that the area of an ellipse with semi-major axis a and semi-minor axis b is pi * a *b.

How exactly do I find a and b in terms of h? I know I need to do something with linear relationships but I've forgotten the method. Is it something to do with y = ax+b?