V = d/t only if there is constant acceleration. It this case, it is not.
d=1/2at^2 only if v_initial is = 0. This is also not true.
More relevant equations are:
vf^2 = vi^2 + 2ad
d = 1/2at^2 + (vi)t
However, in both equations you still have two unknown which will yield infinite...