Ball thrown upward and dropped ball at same height

[Pict]

A ball is thrown upward from the ground with a initial speed of 25 m/s; at the same instant, another ball is dropped from a building 15 m high. After how long will the balls be at the same height?

I'm not even sure how to start it so help!

 

[Kalie]

Draw a picture, with the applied forces.

 

[robphy]

First, consider just one ball.
Can you determine its motion as a function of time?

 

[Pict]

I'm not sure what formula to use to determine the time and I drew a pic but it didn't really help.