9^n+3^2n+1=8
3^2n + 3^2n = 7
2 * 3^2n =7
3^2n = 7/2
原式=[3^(3n*2)] - 6 * (3^2)^2n
=(3^2n)^3 - 6 * (3^2n)^2
=(7/2)^3 - 6*(7/2)^2
=-245/8