Finding velocity use energy conservation help

[xbebegirlx]

A .145kg drop from sixth story building window. If each story is 2.5m tall, use energy conservation to find the rock velocity when it strikes the ground. (ignore air resistance)

I have:
mass .145kg
h 2.5m each story
2.5m * 6 = 15m (total height)
gravity 9.8ms^2
V=?
I am confuse, what is energy conservation?
please help me to start this

 

[icystrike]

mgh=0.5mv^2
2gh=v^2
v=sq root(2gh)

 

[xbebegirlx]

thanks got it