Mass on a spring from equilibrium

[mancity]

Homework Statement

An object with mass m is suspended at rest from a spring with a spring constant of 200 N/m. The length of the spring is 5.0 cm longer than its unstretched length L, as shown above. A person then exerts a force on the object and stretches the spring an additional 5.0 cm. What is the total energy stored in the spring at the new stretch length?

Relevant Equations

Fs=1/2kx^2

Can someone explain that, when using the formula (Fs=1/2 kx^2) why do we use x=0.1m instead of 0.05m? Seems like a simple concept but why isn't it 0.05m (since 0.05m from equilibrium). Thanks.

 

[nasu]

If you use the formula $E=\frac{1}{2}kx^2$, the reference for energy (zero energy) is the position with the spring unstretched. So, you need the extension of the spring relative to the unstretched position.