Calculating Potential Energy Increase in a Massless Spring

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Homework Statement

A massless spring hangs from the ceiling. How much does its potential energy increase, if a 28.8 kg mass is attached to it? The spring constant is 4800 N/m.

The Attempt at a Solution

I set 0.5Kx^2=mg. I then divided the spring constant by mg. I got 17 J. This really doesn't seem right to me. Any import would be greatly appreciated

 

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-kx = mg

not 0.5kx^2=mg

find x and then proceed

 

[preluderacer]

Ok so I solved for x and got 0.0588 meters. I then plugged it into the 0.5kx^2 and got 8.3 J. Does this seem correct?