Fluid pressure problem

Machodog

1. The problem statement, all variables and given/known data
A uniform capillary tube (with one end sealed) contains air trapped by a mercury thread 40mm long. When the tube is placed horizontally, the length of the air column is 36 mm. When placed vertically, with the open end of the tube downwards, the length of air column is now x mm (the mercury is still 40mm).

Calculate x if the atmospheric pressure is 760 mmHg, assuming that the air obeys Boyle's law.


2. Relevant equations
Boyle's law, pV = constant.
density = mass/volume perhaps?

3. The attempt at a solution
I really have no clue how to start, any hints would be great.