5y^n-2y^(n+1)+6y^n-7y^(n+1)-3y^n
=(5y^n+6y^n-3y^n)+[-2y^(n+1)-7y^(n+1)]
=（5+6-3）y^n+(-2-7)y^(n+1)
=8y^n-9y^(n+1).