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.