The depth of water behind the hoover dam is 220 m. find the water pressure at the base of this dam.

Pressure under water is called the gauge pressure.

Now the next part is difficult to put into an equation because you need the symbol for density, which is ρ, pronounced rho.  I cannot type ρ into the equation so I will use p.
(m=mass, V=Volume)

A=Area of the base of the object, or cylinder of water above the object, h= height of object

The "A"s cancel, leaving us with


You are only looking for the water pressure, so you would not be concerned with atmospheric pressure on top of that.



The density of water is 
Use 1000 for ρ
Gravity is 9.8
Height is 220


(Pa is Pascals, the unit for pressure)