How Do You Calculate the Energy Stored in a Nonlinear Spring?

[Charanjit]

1. Homework Statement

The force exerted by an unusual spring when it's compressed a distance 20cm from equilibrium is given by F= -kx-cx³, where k= 233 N/m and c= 4.0kN/m³

Question: Find the energy stored in this spring when it's been compressed 20cm.



2. Homework Equations
Equation already given, I think we supposed to use that one. But the equation of spring constant is U_s= (1/2)kx₀²-(1/2)kx_f².



3. The Attempt at a Solution .
I actually just put in the right numbers in the equation given, but was incorrect. Did integrate the function, and was wrong. No clue what to do.

 

[Nebozilla]

Force doesn't equal energy. Hope that helps haha :)