How do I calculate the time of flight for a horizontally fired cannonball?

[needhelpfast]

Time for a Projectile - Please Help!

Homework Statement

You have traveled to a distant planet named Goolack and brought along a large cannon. You set the cannon on a high cliff on Goolack and you fire a cannonball horizontally off the cliff into a methane ocean. The cannonball follows a projectile motion until it splashes into the methane. Which of the following factors will determine the time that the cannonball is in flight? Goolack is too small to hold an atmosphere and therefore you can ignore atmospheric ("air") resistance.

true/ false
the mass of the cannon ball
the initial velocity of cannonball
the density of the cannon ball.
Goolack's acceleration of gravity
the height of the cliff on Goolack
the radius of the cannon ball

Homework Equations

H=v(0y)*t-1/2gt^2 i think?

The Attempt at a Solution

i think that it only depends on the acceleration due to gravity and the height of the cliff, but i am really not sure

 

[needhelpfast]

Please help! This is due really soon!

 

[ehild]

Your formula is valid if you take the place where the cannon is fired as y=0 and H, the height of the ocean negative.
You fire the cannon ball horizontally, so what is the value of v_oy?

Look at the formula. What would you need to know to get the time of flight? Are other quantities which do not appear relevant?

ehild