Answer:
 1.5 m/s
Explanation:
Conservation of momentum means the momentum of the system before the collision is the same as after.
The before, after momentum of each ball is ...
 5 kg ball: (5 kg)(2 m/s), (5 kg)(-1 m/s)
 10 kg ball: (10 kg)(0 m/s), (10 kg)(v)
The sum of the "before" products is the same as the sum of the "after" products:
 (5 kg)(2 m/s) +0 = (5 kg)(-1 m/s) +(10 kg)v
 (10 +5) kg·m/s = (10 kg)·v . . . . . add (5 kg)(1 m/s) to both sides
 v = (15 kg·m/s)/(10 kg) = 1.5 m/s
The speed of the larger ball will be 1.5 m/s. Its direction of motion will be the opposite of that of the 5 kg ball after the collision.
