Finding period of balls attached with spring

[atim]

Homework Statement

If there are 2 balls attached with a spring, how to calculate period?

Relevant Equations

T=2pi*root(L/g) , T=2pi*root(m/k)

Which formula do I have to use? and why?

 

[haruspex]

Problem Statement: If there are 2 balls attached with a spring, how to calculate period?
Relevant Equations: T=2pi*root(L/g) , T=2pi*root(m/k)

Which formula do I have to use? and why?

It is of no help to know a formula if you do not also know the context in which it applies.
Can you specify the contexts for your relevant equations? If the two balls on a spring system does not fit those, can you see how some part of the system does? Hint: symmetry.