Solve for elapsed time

[roadrunner]

Solve will do it if you use exact values. In CAS:

d:=(vi,t,a)->vi*t+(1/2)*a*t^2

then:

USIMPLIFY(solve((exact(d(18.51_(m/s),t,11.17_(m/s^2)))) = (exact(79.8_(m))),t))

gives:

{2.47014692829_(s),−5.78438148514_(s)}

-road