Three point charges $q , - 2 q$ and $2 q$ are placed on $x$ axis at a distance $x = 0 , x = \frac { 3 } { 4 } R$ and $x = R$ respectively from origin as shown. If $q = 2 \times 10 ^ { - 6 } \mathrm { C }$ and $R = 2 \mathrm {~cm}$, the magnitude of net force experienced by the charge $- 2 q$ is $\_\_\_\_$ N.