Water jet from air pressure: find velocity

[^DsOoMmI^]

Homework Statement

it told: 1. Water is forced out of a fire extinguisher by air pressure
2. the water level in the tank is 0.500 m. below the nozzle
3. find gauge air pressure in the tank (above atmosphere) for water jet to have speed of 30.0 m/s

Homework Equations

i don't know what equation i need to use ...!
it ask: How much gauge air pressure in the tank (above atmosphere) for the water jet to have speed of 30.0 m/s

The Attempt at a Solution

I don't know how to do it ... it give me only
gauge air pressure in the tank (above atmosphere)

 

[Dick]

You really need to have at least SOME thoughts on the physics before anyone can help. I'll give you some starting points, i) how much energy in a mass traveling at 30 m/sec? ii) how much energy to raise a mass 0.5m? iii) how much energy does a mass gain coming out of the high pressure environment inside the bottle (perhaps another property of the body is more important than the mass)? How are these three energies related? Surely you can come up with some formulas that are relevant.