step-1:"find distance between the point of(3,-1) and (-3,7)" "distance between two point can be find:" r^2=Delta x^2+ Delta y^2 r^2=(3+3)^2+(-1-7)^2 r^2=9^2+(-8)^2=81+64=145 step-2:"use Coulomb's law" F=(k*q_1*q_2)/r^2 F=(9.10^9*3*(-2))/145 F=(-54*10^9)/145 F~=-0,372.10^9N