I have been asked to write d2s/dt^2 = −s as first order system, or vector field, on the phase plane.

and then compute its potential, I have no idea how to do either.

I have looked it up, I am currently on this website Pauls Online Notes : Differential Equations - Systems of Differential Equations and still don't understand. It's not in my course notes either.

Please help if you have any idea, I have an exam in a couple of week and NEED to know this.