According to given question, change in momentum of the ball
`Deltap=p_(f)-p_(i)=m(v-u)=15xx10^(-3)[20-(-12)]`
So, by impulse- momentum theorem, impulse `I=Deltap=4.8Ns`
And by time averaged definition of force in case of impulse
`F_(av)=(I)/(Deltat)=(Deltap)/(Deltat)=(4.80)/(0.01)=480N`