y(x)=∑ n=0
[infinity]
​
a n
​
x n
be the series solution of a differential equation. The coefficient recursive relation is: (n+2)(n+1)a n+2
​
=3(n+1)a n+1
​
−2a n
​
,n≥0. Let b n
​
=a n
​
− n
1
​
a n−1
​
,n≥1. Then b 8
​
b 6
​
​
=