[StevieG]

(4 2) or C(4 2) is a specific case of the function C(n k) (often read as "n choose k"). If you have n distinct objects, and choose k from that set, and the order does not matter, the function tells you how many different combinations of k items you get:

C(n k) = n!/(k! * (n-k)!)

here ! indicates factorial
n! = n * (n-1) * (n -2) * .. * 1

in other words, multily all thenumbers from 1 to n together

so C(4 2) = 4!/(2! * (4-2)!) = 4*3*2*1/(2*2) or 6.