Equation for calculating periapsis from apoapsis?

[goduranus]

Recently I had been thinking about debris falling into a gravity well and entering orbit, and I was wondering where can I find the equation that calculates apoapsis altitude and velocity from periapsis altitude and velocity and vice versa, around the Sun for example?  I am not from a physics background, so I didn't know where to look,

Edited May 26, 2016 by goduranus

[kurja]

https://en.m.wikipedia.org/wiki/Vis-viva_equation

[goduranus]

Thanks!