Yes, I get 28 too.
In all these questions it's best to work out what pieces of information you have and which formulas you can use.
If you look at the first second (i.e when u=0 and t=1 ) you can deduct that for the first second v=a.
If you look at the second second (i.e when u = a from the last step), t is again 1 and you know S. That is enough information to be able to work out a and once you know this you can complete the question