Answer: 3.2 * 10^-3 C
Explanation: In order to solve this problem we have to use the Coulomb force given by:
F=k*q^2/ d^2 where he consider the same charge for each point
so, we have
q^2= F*d^2/k= 1.137* (10 C*9*10m)^2/9*10^9 N*m^2/C^2)=3.2 * 10^-3 C.