Physics questions---------------------…

there's an astronaut that drifts 10m from the spaceship. to get back to the ship he throws a 2.5kg box, which had a acceleration of 8.0m/s^2 for 0.5s. Because of action reaction, the force from throwing the box also propels him back to the ship at 20N. How do you work out what speed he travels back to the ship by?