Can someone please explain the method to solving the following electrostatics problem? Thank you very much.

Consider three charged sheets, A,B, and C. The sheets are parallel with A above B above C. On each sheet there is a surface charge of uniform density: –4 x 10^(-5) C/m^2 on A, 7 x 10^(-5) C/m^2, on B, and –3 x 10^(-5) C/m^2. (The density given includes charge on both sides of the sheet.) What is the magnitude of the electrical force per unit area on each sheet? Hint: Check to see that the total force per unit area on the three sheets is zero.