At what distance along the central axis of a uniformly charged plastic disk of radius r = 0.400 m is the magnitude of the electric field equal to 1/5 times the magnitude of the field at the center of

At what distance along the central axis of a uniformly charged plastic disk of radius r = 0.400 m is the magnitude of the electric field equal to 1/5 times the magnitude of the field at the center of the surface of the disk?