Impulse=(force) x (time of the force)
Also ...
Impulse = change in momentum of the body that got hit by the impulse
For this question, you don't know the force during the bounce off the wall OR how long it lasted. This is a pretty good indication that the first formula for the impulse won't be very useful. So you'll have to use the second one ... change in momentum of the ball before and after the impulse. (Remember: momentum = m•v, and v has direction).
Initial momentum = mv = (2kg)(6m/s) = 12 kg-m/s away from you.
Final momentum = (2kg)(4 m/s) back = 8 kg-m/s toward you.
Change in momentum = 20 kg-m/s toward you. THAT'S the impulse that the wall delivered to the ball.
