Assuming constant acceleration, the goalie slows the ball from 18 m/s to rest in 0.035 s, so that the acceleration felt by the ball is
[tex]a_{\rm ave} = a = \dfrac{18\frac{\rm m}{\rm s}-0}{0.035\,\mathrm s} \approx \boxed{510\dfrac{\rm m}{\mathrm s^2}}[/tex]