controls:

acceleration voltage Va
plate voltage Vp
Real experiment about deflection of electrons in electric field of a plate capacitor
grid for measurement
${y(x) = \frac {V_{\text p}}{4\cdot \text{d}\cdot V_{\text a}}\cdot x^2}$

Task:
Calculate the plane capacitor voltage so that the electron beam and the plot matches.
Calculate, don't guess!