A small plastic ball of mass m = 3.00 g is suspended by a string of length L = 19.5 cm in a uniform electric field, as shown in the figure below. If the ball is in equilibrium when the string makes a θ = 17.0° angle with the vertical as indicated, what is the net charge on the ball?

Answer 1

I assume that E is horizontal
look at tension T in string
force up = mg
force sideways = k Q E
string does that with tension
so
T cos 17 = mg
T sin 17 = k QE
sin/cos = tan
so
tan 17 = k Q E/mg