Set Notation (GCSE Standard)

Hi,
I need help with a set notation question, bear in mind I am doing GCSE, and if possible could you explain to me why, Thanks.

P and Q are two sets
n(P) = 9 and n(Q) = 5

Find the value of n(P u Q) when P n Q = a null set

Can you also explain what n(P) or n(Q) means, as I am sure that I try and work it out if I knew what they said.

Thanks

n(P) means the number of members of the set P and similarly for n(Q).

n(P U Q) = n(P) + n(Q)
if P and Q have no members in common which is what P n Q = null means.

Edit. And more generally n(P U Q) = n(P) + n(Q) - n(P n Q)

P.S. It's a pity I can't easily do the intersection symbol and have to use n for two different things.