What is the difference between the simple formula d=st and d=(initial velocity)t-8t(sq)

d = ut + \frac{1}{2}at^2 takes uniform acceleration into account.

Thanks for the reply but that now leaves me with a third formula. Yours was close to mine but not exact, was wondering what is the diff between the 2. When would you use one and not the other?

Yours has -8 instead of 1/2a. I assume this is a special case.