Find launch speed given spring constant and compressed distance

[Chica1975]

Homework Statement

A potato gun contains a spring which is compressed a distance of 7.30cm (0.073m) when it is loaded. If the spring's constant is 41N/m, what is the launch speed of a .00321kg piece of potato when the gun is fired? Give the answer in m/s.

Homework Equations

Power = W/T
V^2=Vi^@+2a(y-y0) (projectile motion)
I have no idea what else I need - am stuck.

The Attempt at a Solution

I drew what it might look like and tried to work out mg (0.00321*9.8 = .31458) as the height but am really stuck.

the lecturer has given the answer as 8.3m/s

Help!

 

[Stonebridge]

Equate the energy stored in the spring to the kinetic energy of the potato piece.
You should know a formula for the energy in the spring in terms of the spring constant and the compression.