Virtual Work-very confused about the sign conventions

[doubled]

The problem statement is "each of the four uniform links has a mass m. determine the horizontal force P required to hold them in place in the vertical plane as shown." I attached the solution, but I don't get when a force causes a +/- virtual work.

 

[TaxOnFear]

Erm, I'm not sure how work would come into this problem? What exactly are you asking?