Combination ~ ~ ~ means there is no possibility of sequential selection ~ ~ ~

For example, how many possibilities are there to choose two out of four books, which can be expressed by C up 2 down 4.

(4*3)/(2* 1)=6

That is (9*8)/(2* 1)=36.

Is the factorial of the following numbers (the upper ones and the lower ones), and then, just divide by the upper factorial ~ ~ ~