I would like to ask part b thanks

6. [18] (Based on Tipler \& Mosca 6th edition - Chapter 22, problem 12) Starting from the electric field strength and electric potential on the z-axis due to a uniformly charged disk of radius R with surface charge density lying on the xy-plane (Lecture notes Chapter 2, Page 15; Chapter 4, Page 13-14), we checked in lecture that at large distances (z>) the system behaves like a point charge R2, while at a distance very near the disk (z