Ender and Shen are flying at each other during a battle in space. Ender moves with a velocity v1 of 12 m/s and Shen, with a mass of 45 kg, moves at a velocity v2 of 9.0 m/s. When the two collide, they hold on to each other and move with a velocity v3 of 6.4 m/s. What is Ender’s mass, in kg?