A 3 kg ball with an initial velocity = (4.00i + 3.00j)m/s collides with a wall and rebound with a final velocity = (-4.00i + 3.00j)m/s. The ball is in contact with the wall for delta t =2.00E-3s. (specify answers with unit vectors)

a) what is the change in the ball's momentum? ans -24i + 0j

b)what is the impulse exerted on the ball by the wall?
ans mvf - mvi = 0 - (-24i) = 24 kgm/s (is this correct?)

c) what is the average force on the ball by the wall?

ans Favg = impulse/t
so: Favg = 24kgm/s / 2E-3 = 1.2E4 (newton?)

d) what is the average force on the wall by the ball?

??

What is your question? THe last part, is opposite sign to c).

so it appears, I was on the right direction? therefore both a) b) c) are correct and that for d) I'll have opposite negative response to C.

yes, they are all correct.