Derivation of a lagrangian from the equations of the motion

Question

We know that if we have a lagrangian, then we can get the equations of the motion using the Euler-Lagrange equations. My question is about the converse of this statement: If one knows the equations of motion, like $x = 0$ and $2y'' + 3 y^2 = 0$, then how to derive the lagrangian ( or equivalent lagrangians) ?