Question 1066052
It is 7C5 and that is 7!/5!2!.  Notice that the two on the bottom add up to the number on the top (ignore the factorial part)
7!=7*6*5!=42  and 42/2 is 21.
5!2!= --   5! 2!=2; the 5! cancel each other.

Any factorial can be stopped anywhere.  n!=n*(n-1)!= n(n-1)(n-2)! and so forth.
Notice that 7C5 and 7C2 are the same.  
0!=1.  This comes from the exponential function, whose integral defines factorial functions.  It is not an arbitrary designation.  
There are 21 ways.