Hi, please help me understand that practice problem.

Cell membranes (the walled enclosure around a cell) are typically about d=7.5nm thick.

They are partially permeable to allow charged material to pass in and out as needed. Equal but opposite charge densities build up on the inside and outside faces of such membrane, and these charges prevent additional charges from passing through the cell wall. We can model a cell membrane as a parallel plate capacitor, with the membrane itself containing proteins embedded in an organic material to give the membrane a dielectric constant of about 35.

What is the capacitance per square centimeter of such a cell wall?

c=