The particle of mass0.5kg was moving with constant speed parallel to the wall. After being acted upon by an impulse of 2N.s in the direction perpendicular to its motion it would gain a velocity 2/0.5 m/s= 4m/s which would be normal to the wall. After collision with the wall its velocity of recede was 3sqrt2 m/s.
Hence coefficient of restitution = (3sqrt2)/4=1(approx)
