7C3 is "7 choose 3" (The "C" stands for Combinations, but "choose" is easier to remember) where the order doesn't matter (7P3 is where order matters).
aCb = a!/(b!(a-b)!), so 7C3 = 7!/(3!(4!)) = 7*6*5*4*3*2*1/(3*2*1*4*3*2*1) = 35
4.5m questions
5.9m answers