Work to Stretch a Spring?

An ideal spring is fixed at one end. A variable force F pulls on the spring. When the magnitude of F reaches a value of 37.5 N, the spring is stretched by 15.6 cm from its equilibrium length. Calculate the additional work required by F to stretch the spring by an additional 13.9 cm from that position (in J).
Update: If you could please explain why you chose to use specific formulas, that would be very helpful! I'm trying to learn how to better solve these problems on my own.
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