A dielectric slab of thickness d is inserted in a parallel plate capacitor whose negative plate is at x = 0 and positive plate is at x = 3d. The slab is equidistant from the plates. The capacitor is given some charge. As one goes from 0 to 3d

After the introduction of dielectric slab, direction of electric field remains perpendicular to plate and is directed from positive to negative plate.

                                   Electric field in air $=\frac{\mathrm{\sigma }}{{\mathrm{\epsilon }}_0}$

and                    electric field in dielectric $=\frac{\mathrm{\sigma }}{{\mathrm{K\epsilon }}_0}$

Positive plate is at higher potential and negative plate is at lower potential. So, electric potential increases continuously as we more from x = 0 to x = 3d.