initially spring is compressed by distance x0 from equilibriuim position at this compression block is given velocity (3k/m)^1/2 x0 so that compression increase and block start shm spring constant =k equation of motion is.

i got how amplitude came but not phase epoch.

ans.y=2x0sin[(k/m)^1/2t+ pi/6]

Each of these constants carries a physical meaning of the motion: A is the amplitude (maximum displacement from the equilibrium position), ω = 2πf is the angular frequency, and φ is the initial phase. By definition, if a mass m is under SHM its acceleration is directly proportional to displacement.