A system consisting of two non-conducting hollow sphere of radius R and 2R respectively separated by distance d=6R (center to center).If +q and -q charges are distributed uniformly over the surfaces of these sphere then electrostatic potential energy of the system comes out `(7kq^(2))/(NR)` .Find the value of `(N)/(5)*(k=(1)/(4 pi varepsilon_(n)))`