The variation of potential with distance R from a fixed point is as shown below. The electric field at R = 5 m  is

Intensity at 5 m is same as at any point between B and C because the slope of BC is same throughout (i.e., electric field between B and C is uniform). Therefore electric field at R = 5 m is equal to the slope of line BC hence byand at R = 3 m potential is constant so E = 0.