Finding speed of a rock, given height acceleration and velocity

I have no idea how to do this question, it was on a test so I dont know the correct answer and when the prof gave out the solutions for the test I was away, so any help with this would be greatly appreciated :)

A rock is thrown upward from the top of a 100m building at 10 m/s. At what speed will it hit the ground (the acceleration due to gravity is -9.8 m/s^2)

I know this is probably like the easiest question ever but I, having trouble seeing what to do.