What is the energy stored in such a cell membrane if the potential difference across it is 7.05*10-2 v?

Capacitance

C = e A/d

or C=Q/V

Energy stored in the capacitor

W=0.5CV^2

This a straight forward problem

W=0.5 e A/d (V^2)

W=0.5 x 4.5 x 4.75 x 10^-9 m^2 / 8.5 x 10^-9

m=

W = ...

since d is the thickness and the energy is inversely proportional to energy stored in the capasitor then with the increase of the thickness of the membrane the energy will decrease.