Math Help - Help with finite quene!!!

A = mean arrival rate of customers per minute
S = the mean service rate per minute
p(n) = steay state probability, n being the total number of customers

ap(n-1) - (a+s)p(n) +sp(n+1) = 0 for n = 1, 2 ,3..., M-1

M is the max number of customers in system

initial conditions are: ap(o) - sp(1) = 0
m∑(n=0) p(n) = 1

for a finite quene we do not require s > a

PLz help, totlally lost, what! freakin insane question