Question: How do I find the unknown X when I know Y?

well first of all this site was very useful for doing my homework, but there is still something I didnt find on it and im sure other people may find it useful.

I have a system of 5 ode's and 6 initial conditions that ive solved successfully and plotted the graphs i need:  Position VS Time

          Speed VS Time

The only thing I initially know is the final position, which i can read on the first graph and note its corresponding time, then refer to this time in the second graph to get its final speed.

Id like to be able to find the exact value of the unknown X (time) with the known position Y.

I know there is command that looks like "f(5)" which returns Y, but in my case i know what Y is and dont know that X is.

hope im clear enough...

what would be the easiest command to use to find the exact value of the unknown X ??

 let me know if i should paste my Maple code also.

Thanks in advance,

Mark!