Troubleshooting a Virtual Work Problem

[curtain]

https://www.physicsforums.com/attachment.php?attachmentid=65907&stc=1&d=1390369310

I've been stuck on doing my revision on this problem.

The equation of the virtual work, from what I deduce is

[du = 0] -Pdx + mg dh

However the answer given is
https://www.physicsforums.com/attachment.php?attachmentid=65908&stc=1&d=1390369325

I'm not unfamiliar to these types of question as I've done quite a few of them. But, I just couldn't understand how'd the answer arrive to that.

I know P, is the force acting at the hydraulic extension at bc, and mg is the downward force of the car and the platform.

 

[rude man]

You need to relate dh to a change in ds, a differential distance the piston moves. This is just geometry.