**1. The problem statement, all variables and given/known data**
**2. The attempt at a solution**
a) [tex]d = \frac{f-f_1}{k}[/tex]

[tex]d = (f-f_1)d = \frac{(f-f_1)^2}{k}[/tex]

Correct?

b) Useful work at max. when [tex]f_1=0[/tex]; [tex]Work = \frac{f^2}{k}[/tex]

Hm...What's wrong here?