combin(x, y)

x!
y!*(x-y)!
^^ division

Where
z! is a factorial
(z)*(z-1)*...*2*1

so 5! = 5*4*3*2*1

Sample:
combin(50,3)

50!
3!*47!

simplifies to

50* 49* 48
3*2

117600
6

19600