A block attached to a spring, pulled by a constant horizontal force, is kept on a smooth surface as shown in figure. Initially, the spring is in the natural length state. The maximum positive work that the applied force F can do is (given that string does not break) : 

\(\frac{2F^2}{k}\)

Applying Work-Energy Theorem :

\(Fl - \frac{1}{2}kl^2 = 0\)

\(l = \frac{2F}{k}\)

Work done : \(Fl = \frac{2F^2}{k}\)