I fail.

Fuck. This problem is getting on my nerves. I can't for the life of me figure out something as simple as this... Maybe because ever time I see my physics book I think of choking the teacher but that's a whole other story. I know it's a really dumb, cliche  problem but I have no idea where to start on it so can you guys help me out?
A ball is thrown upwards with an initial velocity of 25m/s and at the same instant another ball is dropped from 15m high. how long will it be before they are at the same height?

Any help?
Yes, I suck cocks at physics.

Thanks, dude.
I couldn't find those equations in the book. I guess they expected us to derive them ourselves.
Again, thanks.