Answer
The potential difference between $x_i = 1.0~m$ and $x_f = 3.0~m$ is zero.
Work Step by Step
We can write an expression for the potential difference:
$\Delta V = -\Delta x~E$
The potential difference is equal to the the negative of the area under the E versus x graph. We can find the area under the E versus x graph between x = 1.0 m and x = 3.0 m:
$area = \frac{1}{2}(1.0~m)(100~V/m)+\frac{1}{2}(1.0~m)(-100~V/m)$
$area = 0$
The potential difference between $x_i = 1.0~m$ and $x_f = 3.0~m$ is zero.
