MHB Coefficent in a infinite power series

[pac1337]

How do I find a coefficent of x^9 in a power series like this:

 

[HOI]

For (a) 9= 0+ 9= 1+8= 2+ 7= 3+ 6= 4+ 5= 5+ 4= 6+ 3= 7+ 2= 8+ 1= 9+ 0.
There are 10 products that give $x^9$ so the coefficient is 10.