How to calculate this coefficient

How to calculate ak,n,p using this egality ?

[jenny jenny]

heeeellpppp :'( :'(v pleaaaaaase

[John BG]

why double comma after ak?

supply more details, can x be complex?

is this a probability function or CCDF?

[jenny jenny]

x can be 0->20

but I think I put x=1

[jenny jenny]

in the article they say that x can have the value 0,1,2.........20

[jenny jenny]

anyy help