Why is the calculated change in PE for a mass on a spring incorrect?

[Emilyjoint]

Homework Statement

A spring of length 75cm has a mass of 0.5kg hanging from it.
If the spring constant = 40N/m calculate:
a) the extension of the spring
b) The change in PE of the mass

Homework Equations

Part (a) is easy x = (0.5 x 9.81)/40 = 0.123m
Part (b) energy = 0.5kx^2 gives 0.30J which is WRONG !

WHY ?

The Attempt at a Solution

 

[tiny-tim]

Hi Emilyjoint!

A spring of length 75cm has a mass of 0.5kg hanging from it.
If the spring constant = 40N/m calculate:
…
b) The change in PE of the mass
…
part (b) energy = 0.5kx^2 gives 0.30J which is WRONG !

that's the energy of the spring

(energy is not conserved … if you released the mass from rest, it would sail through that equilibrium position at high speed, wouldn't it? )