relative !

A cart is moving along x - direction with a velocity of 4 m/s. A person on the cart throws a stone with velocity of 6 m/s relative to himself. In the frame of reference of the cart the stone is thrown in y-z plane making an angle of 30 degree with the vertical z axis. At the highest point of its trajectory the stone hits an object of equal mass hung vertically from the branch of tree by means of a string of length L . A completely inelastic collision occurs in which the stone gets embedded in the object. Find L such that Tension in the string becomes zero when string becomes horizontal during subsequent motion of the combined mass . Take g= 10 m/s^2