Finding Initial Velocity of a Rock Kicked off a Cliff

[saber1357]

Homework Statement

A soccer player kicks a rock horizontally off a 40m high cliff. If the player hears the sound of the crash 3.00s later, what was the initial speed given to the rock? assume the speed of sound in air is 343m/s.

Homework Equations

Position of Y = -40m
Time = 3s
Position = Ut+.5at^2

The Attempt at a Solution

To find the initial velocity of the rock, I know I must first find the displacement it covered in the x axis. However, I am not sure how to continue. I can use the equation Sx = U*t, but I do not know Sx or U(initial velocity).
How can I derive initial velocity?

 

[Doc Al]

Hints: How long did it take for the rock to hit the ground? How long did it take the sound to travel back to the player?

 

[D H]

Another hint: The rock's initial horizontal does not come into play in determining how long it takes the rock to hit the ground. The initial horizontal velocity only changes how far the rock is from the cliff at the time the rock does hit the ground.