Answer
The electric potential at $x = 2.0~m$ is $~~30.0~V$
Work Step by Step
We can use Equation (24-21) to find the potential difference between two points:
$\Delta V = -E~\Delta x$
The area under the $E$ versus $x$ graph is the negative of the potential difference.
We can find the area under the graph between $x=0$ and $x = 2.0~m$:
$Area = \frac{1}{2}(-20.0~N/C)(2.0~m)$
$Area = -20.0~V$
The potential difference between $x=0$ and $x = 2.0~m$ is $\Delta V = 20.0~V$
Since the electric potential at the origin is $10~V$, the electric potential at $x = 2.0~m$ is $30.0~V$