Finding the voltage at a radius of a cylinder

[Blockade]

Homework Statement

Homework Equations

V_ab = ∫ E*dr

The Attempt at a Solution

[/B]

For part (b) I am not sure if I should set the upper and lower bound of the integral from "R" to "2R" or "2R" to "3R". If done so, this would give me V_2R = V_ab * ln|3R/2R| / ln |3| instead of what is on the image.

 

[kuruman]

The problem is asking you to find the potential difference (i.e. voltage) between a point on the inner cylinder at r = R and a point at r = 2R. What do you think the limits of integration ought to be?