Why is this 'so wrong'? Aside from your potential lack of brackets, which isn't all that uncommon, your answer seems to be either correct or close to correct.
Either way did you mean [tex]a=\frac{s-ut}{0.5 t^2}=2 \frac{s-ut}{t^2}[/tex]?
If so your answer is correct. If you are however only dividing ut by 0.5t^2 it is not. So make sure that when you divide by an expression on one side you divide through the whole expression on the other side as well.