Field of a point charge

A small plastic ball of mass 20 g hangs from a light string between two vertical parallel plates 0.050 m apart, where the electric field is uniform and horizontal. The left plate is at +200 V and the right plate at −200 V. The string makes an angle of 10° to the vertical, with the ball displaced towards the left plate. What is the magnitude of the charge on the ball? (Take $g = 9.81\,\text{m s}^{-2}$.)