Pressure excerted by a Fermi gas

Show that the pressure exerted by a Fermi gas at absolute zero is

                            p = (2NEf)/(5V)      

where N is the total number of particles, Ef the fermi energy and V is the volume.

((If you need to use the relation for the mean energy of particles obeying Fermi-Dirac statistics at absolute zero temperature, you can just quote it's = 3Ef/5  I derived it earlier in this problem, so you don't have to derive it again))