A mass M is lowered with the help of a string by a distance h at a constant acceleration g/2. The work done by the string will be : 

\(\frac{-Mgh}{2}\)

Tension in the string : 

\(T = M(g - a) = M(g - \frac{g}{2})\)

\(T = Mg\)

W = force x displacement = \(-\frac{Mgh}{2}\)

Negative sign is because tension is in the upward direction and displacement is in the downward direction.