Lag1989 problem number 13.10

A large metal plate is vertical and grounded. At a distance of a = 10 cm from the plate, there is a fixed point, to which a ball of mass m = 0.1 g is suspended on a thread with a length of L = 12 cm. When a charge q is imparted to the ball, it will be attracted to the plate, as a result of which the thread will deviate from the vertical by an angle α = 30 °. Find the amount of charge q of the ball.