Solving Spring/Work Problem: Find Min. Work Required

[Tabe]

The exact problem is:
A massless spring is between a 1-kilogram mass and a 3-kilogram mass, but it 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.

I am totally lost on this problem. I mean, I understand what it is asking for, but I don't know which equation to use and what variables to solve for. I know the equation for work is W= 1/2kx^2 but how do I solve for either variable?
I was looking through my notes and saw several formulas that I could use, but each one is missing the same varible, either force or the distance.

Can someone please point me in the right direction...

 

[Pyrrhus]

Consider conservation of Mechanical energy, when the spring is compressed with mass of 3 kg, then there's only spring potential energy, when the mass is released with a speed of 10m/s then there's only kinetic energy. Use both initial and final case, to find the Work that should be done on the spring or the change in spring potential energy.