Solve for elapsed time

[DrD]

With these provided values, (including the units), how could the prime be made to find the elapsed time?

[CAS]
vi:=18.51_(m/s);
d:=79.8_(m);
a:=11.17_(m/s^2);
t=?

d=vi*t + 1/2 * a*t^2;

(Of course, the units could be removed, using left(var), and a mathematic result obtained; however it's the physical result that's desired).

-Dale-