A block having mass m travels along a horizontal frictionless surface with speed vx. What impulse must be delivered to the mass to reverse its direction and continue to travel with a speed of vx

If I had to guess I would say this is what the question fully looks like:

A block having mass m travels along a horizontal frictionless surface with speed vx. What impulse must be delivered to the mass to reverse its direction and continue to travel with a speed of vx?

A) -mvx

B) -2mvx

C) 0

D) 2mvx  

E) mvx

F) none of the above

G) cannot be determined

Explanation: The answer is

The initial momentum is m vx. So what change do you need to have a momentum -m vx afterwards? -2 m vx, because m vx + (-2m vx) = - m vx

So the correct answer is b.