What is 7C3? In mathematics

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