Final speed after collision with the wall
[tex]v_f = -0.4 m/s[/tex]
before collision the speed of ball initially
[tex]v_i = 0.6 m/s[/tex]
time taken for the collision
[tex]\Delta t = 0.2 s[/tex]
now as per the formula of acceleration we know that
[tex]a = \frac{v_f - v_i}{\Delta t}[/tex]
now plug in all values in it
[tex]a = \frac{-0.4 - 0.6}{0.2}[/tex]
[tex]a = -5.0 m/s^2[/tex]
so acceleration is - 5 m/s/s for above situation