In figure shown, the spring constant is k. The mass of the upper block is m and that of the lower back is 3m. The upper block is depressed down from its equilibrium position by a distance δ and released at t = 0.
The minimum value of δ for which the lower block loses contact with the ground is
4 mg/k
At equilibrium Position x=mgk ; Then we compress δ
At highest point: K(δ−mgk)=3mg or δ=4mgk