Factorial for 2 number

As you explain factorial formula (n+k)! and (n-k)! there is already two number and you give example of n= 5 , k= 2 means n>k here but then you told us we have to use formula of n!/k! if n>k so i am confused here if n>k then we always have to use n!/k! this formula alright? ok but when will be use the formula of these (n+k)! and (n-k)! ? please explain.