Answer
$-200\;\rm V$
Work Step by Step
We know that
$$\Delta V=V_f-V_i=-\int_i^fEdx$$
where $\int_i^fEdx$ is the area under the $E$ versus $x$ xurve.
$$\Delta V=-\text{Area}$$
As we see in the given graph, the area from $1$ m to $3$ m is a right triangle.
$$\Delta V=-\frac{1}{2}(3-1)(200-0)$$
$$\Delta V=\color{red}{\bf -200}\;\rm V$$
