The left side of the figure shows a light (`massless') spring of length 0.350 m in its relaxed position. It is compressed to 72.0 percent of its relaxed length, and a mass M= 0.210 kg is placed on top and released from rest (shown on the right).

The mass then travels vertically and it takes 1.50 s for the mass to reach the top of its trajectory. Calculate the spring constant, in N/m. (Use g=9.81 m/s2). Assume that the time required for the spring to reach its full extension is negligible

Use energy principles: 1/2 k x^2 = 1/2 m v^2. You'll need to determine the velocity required for the mass to reach the top of its trajectory in 1.5s.