Is Equation 2.49 Necessary for Griffith's Problem 3.19?

[ehrenfest]

Homework Statement

Please stop reading this unless you have Griffiths E and M book.
Are we supposed to use equation 2.49 to do this? Isn't that only true for conductors? If not, how do you do the problem without using equation 2.49?

Homework Equations

The Attempt at a Solution

 

[Biest]

Eqn. 3.83 and 3.84 are a better bet