Task:
Replace in $y(x)=\frac{a_{\text{y}}}{2\cdot{v_{0}}^2}\cdot x^2$ the term ${a_y}$ for the acceleration in y-direction with a term, that depands on $V_{\text{p}}$, which can be modified in the experiment.

The acceleration in y-direction of an electron in a plate capacitor is constant. So the general kinematic equations provide a term to replace $a_y$.
$$a_y=\frac{F}{m}=\frac{V_{\text{p}}\cdot e}{\text{d}\cdot m}$$

Useable variables:

x = position on x-axis
Va = acceleration voltage
Vp = capacitor voltage
e = charge of an electron
m = mass of an electron
d = distance between capcitor plates

typing tips:

x² must be typed as x*x
2x must be typed as 2*x

Please note the following :

Useable variables:

x = position on x-axis
v0 = velocity in x-direction
ay = accelertation in y-direction
Va = acceleration voltage
Vp = capacitor voltage
e = charge of an electron
m = mass of an electron
d = distance between capcitor plates