Need help on a physics momentum problem.

A 63 kg astronaut on a spacewalk when his line breaks. He throws a 10 kg O2 tank away from the ship with a v=12 m/s. This propels him back to the ship. Assume his v1=0; what is his v2? I really need someone to show and explain to me how to get the answer step by step. I know it’s probably easy to get the answer but my teacher never gave me a formula. If anyone can explain to me how to do this that would really help because I have a bunch of problems like this but no way to get an answer. It shouldn’t take that long to answer. Any help is greatly appreciated.

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