A polystyrene ball is coated with conducting paint. It has a mass of 5,0 x 10 power of -3 kg and a charge of 4,0 uC. An eclectic field pointing upwards is applied. Determined big the electric field must be for the net force on the ball to be zero.

Well, you have the force of gravity F=mg pulling the ball downward and the electrostatic force F=qE pushing it upwards. Set these equal and solve for E. Where are you having difficulty?