Work to compress spring between 1 kg and 3 kg masses

[edud8]

1981B2. A massless spring is between a 1 kilogram mass and a 3 kilogram mass as shown above, but is not attached to either mass. Both masses are on a horizontal frictionless table. In an experiment, the 1 kilogram mass is held in place and the spring is compressed by pushing on the 3 kilogram mass. The 3 kilogram mass is then released and moves off with a speed of 10 meters per second.
a. Determine the minimum work needed to compress the spring in this experiment.





The spring is compressed again exactly as above, but this time both masses are released simultaneously.
b. Determine the final velocity of each mass relative to the table after the masses are released.



attempt:
1/2mv^2=1/2kx^2
1/2mv^2=150, but idk how to solve for k or x

 

[Chi Meson]

1/2mv^2=1/2kx^2
1/2mv^2=150, but idk how to solve for k or x

You don't need to. You only need to know the amount of energy stored in the spring, which is the 150 J you found.

This is the total KE of both masses in part 2, and you also need to remember something else that is conserved.

Your answer will be a neat integer.