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.
