A parallel plate capacitor with plate distance $5$mm$,$ plate surface area $40$ mm$2$ and air between the plates is placed on the x$-$axis. There is a surface charge density of $20$nC$/$m$2$ on the plates.

a$)$ Determine the electric field and potential between the plates.

b$)$ Calculate the energy stored between the plates.

c$)$ Calculate the electrostatic energy density between the plates of the capacitor using the given values.