Answer
$25\;\rm V$
Work Step by Step
$$\color{blue}{\bf [a]}$$
We can use any software calculator to draw this function, as shown below.
$$\color{blue}{\bf [b]}$$
We know that
$$\Delta V=V_f-V_i=-\int_i^fE_xdx$$
where $E_x=-1000 x$,
$$\Delta V =1000\int_i^fx dx$$
and $x_i=-0.20$ cm while $x_f=0.30$ cm,
$$\Delta V =1000\int_{-0.20}^{0.30}x dx=\dfrac{1000}{2}x^2\bigg|_{-0.20}^{0.30}$$
$$\Delta V =500(0.3^2-[-0.2]^2)$$
$$\Delta V =\color{red}{\bf 25}\;\rm V$$
