Calculating impulse from velocity change

[cjedow]

Homework Statement

A small rubber ball of mass m is thrown at a brick
wall. Initially, the ball has a speed v0 and is traveling perpendicular to the wall. After
the ball hits the wall and rebounds, it is traveling perpendicular to the wall at speed
(2/3)v0. The ball is in contact with the wall for a time Δt. (Assume that Δt is so short
that gravity doesn’t appreciably change the ball’s momentum.)
a. What is the magnitude of the impulse from the force of the wall on the ball?
b. As quantitatively as possible, plot the instantaneous force
exerted by the ball on the wall versus time. On the plot,
indicate the average force exerted by the wall on the ball.

Relevant Equations

J= m * vf - m * vi
J=F * t

I wrote:
J = m(2/3)v0 - mv0 for part a but I'm not sure if that's correct or if I need something else?
I have no idea how to do part b. I wrote F * t = m(2/3)v0 - v0 but I honestly don't know what I'm doing.

 

[haruspex]

Momentum has direction. If it were to hit at speed v and bounce back at speed v there would be no change to the magnitude of the momentum, but the magnitude of the change in momentum would not be zero.
To put it another way, be careful with the signs of v_i and v_f.

For part b, assume it behaves like a point mass with a light uniform spring in front... though that is not strictly true.