Thank you for the quick and detailed response. I had to write it down to understand it but I got it now.
Yeah, that ##v_{2f} = 0## also made me wonder, I will ask other people about it.
Thanks!
So after not being able to solve this problem I did some researching online. I was looking around and came across this video, where they give the following equations for solving for the final velocities of both balls:
v1f=((m1-m2)/(m1+m2))*v1i
v2f=(2m1/(m1+m2))*v1i
I plugged in my numbers and...