Velocity of a particle of mass 2 kg changes from v1→=−2i^−2j^ m/s to v2→=i^−j^ m/s after colliding with a smooth plane surface.

ΔP→=2(i^−j^)−2(−2i^−2j^)=6i^+2j^km/sSo,  ΔP→ makes angle tan−113 with positive x-axis which is also the direction of normal force exerted on the ball. So the plane surface makes an angle π2+tan−113 with positive x-axis.