You need to use conservation of momentum.
Momentum (P) = mv.
Work out the momentum of the oxygen tank. The astronaut has the same momentum. You know his mass and his momentum, so his speed = m/P
I won't give you the answer,but will ask you a question.
Why would the astronaut throw an oxygen tank when all he would have to do is open the valve a little and use the tank as a source of propulsion to get back to the shuttle and then use it to slow down when he gets back to the shuttle?