Can u tell me it's length

A small charged ball of mass m and charge q is suspended from the highest point of a ring of radius R by means of an insulating cord of negligible mass. The ring is made of a rigid wire of negligible cross section and lies in a vertical plane. On the ring there is uniformly distributed charge Q of the the same sign as q. Determine the length of the cord so as the equilibrium position of the ball lies on the symmetry axis perpendicular to the plane of the ring in cm.

( Take Q = q = 90 nano C, R = 5cm, ε₀ as permittivity of space with approximation of 8.85 equal to 8.9) Mass of ball is 1g This is my first question in physics, tell me how it was