As per Newton's II law we know that
[tex]F = \frac{\Delta P}{\Delta t}[/tex]
now here we will have
[tex]F = \frac{m(v_f - v_i)}{\Delta t}[/tex]
we know that
m = 500 g = 0.5 kg
[tex]v_i = -10 m/s[/tex]
[tex]v_f = +8 m/s[/tex]
F = 4000 N
now from above equation we will have
[tex]4000 = \frac{0.5(8 - (-10))}{\Delta t}[/tex]
[tex]\Delta t = \frac{9}{4000} = 2.25 ms[/tex]