Please explain this question. I have not found an adequate explanation

Two uniform links OB and BP are attached/pinned to the

ground at O and the massless block at P. The rod OB

has mass m and length L, while the rod BP has mass m/2

and length L/2, respectively. There is a linear spring of

stiffness k attached to the block at P on one

end and to the wall at the other end. The

system is in vertical plane. The spring is

unstretched when the two rods are horizontal,

that is, ?=0, and OP=3L/2.

Use the Principle of Virtual Work to find the

equilibrium position of the system in terms of the angle ?.

g O P k m, L m L O B 2 2