Assuming north as positive direction, the initial and final velocities of the ball are:
[tex]v_i=-43 m/s[/tex] (with negative sign since it is due south)
[tex]v_f=+51 m/s[/tex]
the time taken is [tex]t=1.0 ms=0.001 s[/tex], so the average acceleration of the ball is given by
[tex]a= \frac{v_f-v_i}{t}= \frac{51 m/s-(-43 m/s)}{0.001 s}=9.4 \cdot 10^4 m/s^2 [/tex]
And the positive sign tells us the direction of the acceleration is north.
