Two infinite plane parallel sheets, separated by a distance $d$ have equal and opposite uniform charge densities $\sigma$. Electric field at a point between the sheets is

Let $E_1$be the field due to positive charged plate and $E_2$ be the field due to negative charged plate

As the fields created by both plates are in same direction, 

Net electric field = $E_1+E_2,$, $\because$Electric field due to infinitely charged sheet = $\frac{\sigma }{2{\epsilon }_o}$

$\therefore$Electric field, $E = \frac{\sigma }{{\epsilon }_0}$