Stuck on physics problem

[lmf22]

I am stuck on these 2 physics problems. Can anyone help?

1) A rock is dropped from a sea cliff and the sound of it striking the ocean is heard 2.5 s later. If the speed of sound is 340 m/s, how high is the cliff?

2) A first stone is dropped from the roof of a building. 2.30 s after that, a second stone is thrown straight down with an initial speed of 35.0 m/s, and it is observed that the two stones land at the same time.
(a) How long did it take the first stone to reach the ground?
(b) How high is the building?
(c) What are the speeds of the two stones just before they hit the ground?

Any help on how to get started would be great. Thanks in advance.

[Parth Dave]

Can you think of an expression to find the time while the stone is falling down? Now can you think of one for the time while the sound is travelling back up to the top of the cliff?

[Kelju Ivan]

2) Do note that the time the first stone takes to fall down is t while with the second stone it's t-2.3s. This should make forming the equation easier.