Gauss Law Problem to determine F/A

[max190us]

Three large but thin charged sheets are parallel to each other as shown in the figure . Sheet I has a total surface charge density of 7.9 nC/m^2, sheet II a charge of -3.3 nC/m^2, and sheet III a charge of 5.4 nC/m^2.

Figure:

----------------(I)
----------------(II)----------------(III)

Estimate the magnitude of the force per unit area on sheet I, II, and III, N/m^2.

**I do not know how to start**

Attempt: E=F/Q; Int(E) dA=Q/epsilon
2. E=F/Q; Int(E) dA=Q/epsilon; \sigma=Q/A
3. I combined the above eq and got this:
\sigma*(\sigma)/(2epsilon)

 

[max190us]

The attempt I have done:

E = σ/2εo

The force per unit area on sheet I due to sheet II is:

F = q’E = σ’AE => F/A = σ’E = σ’(σ/2εo) = 3.3x10^-9(7.9x10^-9)/2*8.85x10^-12

F/A = 1.47x10^-6 N/m²

or
FI/A = -σσ’/εo= (3.26x10^-6),

FII/A = -σσ’/εo + σσ”/εo

FIII/A = σ’σ”/εo


---IT IS NOT RIGHT! HELP ME!