derivation law

[ahmed]

hello all
in derivation variance of hyper geometric distribution i can not understand this line who can help me plz

 

[stapel]

hello all
in derivation variance of hyper geometric distribution i can not understand this line who can help me plz
View attachment 4052

What are the definitions (the expanded forms) of the expressions, such as C(N,n) and C(N-2,n-2)?