23just use Ex= - ΔV/Δx = - (Vx - Vo)/x = constant
Vo = V at origin , Vx = V at x
similarly for y and z
for third, suppose you give Q chargeto outermost shell ,
now the electric field between the shells is independent of Q, so the energy stored in that field is always going to be there and you cant minimize it ,
but the electric field at a pt outside the gr8er shell is due to (Q+q)
so if energy is to be minimized,then we hav to see that energy density is minimum , i.e εE2/2 is minimum , that will be when Q+q = 0