Answer :
You can't ... unless you set s(t) = to some value and then solve the resulting equation for t.
Suppose t^2-4t-12 = 0. This can be factored: (t-6)(t+2) = 0.
Then t=6 and t=-2.
Suppose t^2-4t-12 = 0. This can be factored: (t-6)(t+2) = 0.
Then t=6 and t=-2.