Compute each part of following problem given below:

Problem: A conducting sphere of radius 'a' is surrounded by a concentric conducting spherical shell of radius 'r'. The inner sphere carries a charge -Q while the outer shell carries a charge +Q.

Part A: Find the electrostatic field energy contained in the region between the two spherical surfaces by integrating the electrostatic energy density over this region.

Part B: Find the force as a function of 'r' between the two shells when the inner sphere is held constant and the radius of the outer sphere is expanded.

I want help to find the electrostatic field energy contained in the region between the two spherical surfaces.