Well first, this part: "Because v1>v2, it must be that (m1 +m2) * vf = m1v1 + m2v2 > m1v2 +m2v2 = (m1 + m2) * v2. Thus vf > v2."
I dont understand where they got "m1v2 +m2v2 = (m1 + m2) * v2" ESPECIALLY THE "m1v2" part. How did they obtain that?
I dont really know how they used the law of conservation of momentum properly. Could you please show me an even more detailed step-by-step of the proof they did?