Consider two infinitely large plane parallel conducting plates as shown below. The plates are uniformly charged with a surface charge density $$+\sigma$$ and $$-2\sigma$$. The force experienced by a point charge $$+q$$ placed at the mid point between two plates will be:

Field due to an infinite sheet of charge is

$$E=\frac{\sigma}{2\varepsilon_0}$$

For the left plate (+σ):

Field points away from positive sheet, so at midpoint it is toward right.

$$E_1=\frac{\sigma}{2\varepsilon_0}$$

For the right plate (−2σ):

Field points toward negative sheet, so at midpoint it is also toward right.

Magnitude

$$E_2=\frac{2\sigma}{2\varepsilon_0}=\frac{\sigma}{\varepsilon_0}$$

Net field

$$E=E_1+E_2$$

$$=\frac{\sigma}{2\varepsilon_0}+\frac{\sigma}{\varepsilon_0}$$

$$=\frac{3\sigma}{2\varepsilon_0}$$

Force on charge +q:

F=qE

so 

$$F=\frac{3q\sigma}{2\varepsilon_0}$$