A light spring (k= 130 N/m) is compressed 15 cm between two masses (1.2 kg and 2.4 kg) on a frictionless surface. The system is released and the masses fly apart.

Which mass will have the larger magnitude momentum? Why?
Which mass will have more kinetic energy?
What is the magnitude of the momentum and energy of each mass?

Same. Conservation. Zero before so zero after.

Now we need a system of two eqs and two unknowns:
m1v1 = m2v2
1/2 m1 v1^2 + 1/2 m2 v2^2 = 1/2 k x^2
Solve the first for either v1 or v2 and substitute into the second. When you've found v1 and v2 you can see which will have greater KE (or if you're clever you'll note the smaller will be moving faster and KE is more dependent on v than m so the smaller should).