Collision of two initially moving objects

[Teachme]

So I know how to solve for two initially moving objects that collide one way, but I was reading on this website another way to do it and I am not understanding a step. I've included a picture of the site and underlined the step I don't understand. I've tried a couple ways but I can't get it.

Here is the link the the site as well

http://farside.ph.utexas.edu/teaching/301/lectures/node76.html

Thanks for reading.

 

[Simon Bridge]

"It is easily demonstrated that"... a bunch of equations that result.
YOu should be able to work backwards from the equations - you have to apply the transform to com frame as suggested and then work through the consequences of conservation of momentum.