Problem: A small 12.3g plastic ball is tied to a very light 29.8cm string that is attached to the vertical wall of a room. A uniform horizontal electric field exists in this room. When the ball has been given an excess charge of -1.50 uC, you observe that it remains suspended, with the string making an angle of 17.4 degree with the wall.
Part A: Find the magnitude of the electric field in the room.
Please help, completely confused!! Give steps if possible.