Find the expression the relates total capacitance with the displacement d

Capacitors can be used in sensing applications such as fuel gauges in airplanes and vibration sensors. The following is an application where a capacitive sensor can be used to find displacement d. The area of the plates is 4 cm x 4 cm. S1 = 2.5mm, S2 = 1mm. The area between the plates is filled with air which has a relative permittivity of 1 a. Find the expression the relates total capacitance with the displacement d

b. Find the displacement if the total capacitance is 6.181 x 10^-12 F