Derive the expression for the work done by the ideal spring

[Alexanddros81]

Homework Statement

Derive the expression for the work done by the ideal spring on the slider when the
slider moves from A to B. Assume that the free length of the spring is (a) $L_0 = b$;
and (b) $L_0 = 0.75b$

Homework Equations

The Attempt at a Solution

Can you check if my solution is correct?

 

[Orodruin]

Looks reasonable.