Two Blocks and A Spring

A system consists of two blocks that are connected by a spring. Mass of the two blocks are m and M, and the spring constant is k. The block M is on a horizontal surface, and the another one is hang through a light pulley. See the picture.
Initially the system is at rest and the spring is relaxed. As the system moves by gravitational acceleration g, length of the spring will oscillate.
Determine the period T of this oscillation and find the equilibrium length x of the spring! (Ignore friction)