A horizontal electric field causes the charged ball to hang at a 15o angle from the vertical. The ball weighs 3.0 g and carries 20 nC charge. It is hanged vertically by a non-conducting string of 60 cm length. The ball is attached to a spring which extends horizontally when the ball moves in the electric field. The spring is plastic, so it does not discharge the ball, has a spring constant 0.050 N/m, What is the electric field strength