I get a very large deceleration (-5358.4m/s^2) using two different equations:
1) u=13.8889, v=0, s=0.018, a=?
v^2=u^2+2as 0=13.8889^2+2xax0.018 -13.8889^2/0.036=a=5358.4m/s^2
Are you sure your value for t is right? I used s=((v+u)t)/2 and got 0.00259s
2) u=13.8889, v=0, t=0.00259, a=?
v=u+at 0=13.8889+0.00259a -13.8889/(0.036/13.8889)=a=5358.4m/s^2