>>304
f(n) = 2＾(3n)-7n-1

f(n+1) = 2^(3(n+1)) -7(n+1) -1
= 8 (2^(3n)) -7n -8
= 8 (2^(3n) -7n-1) + 8(7n+1) -7n-8
= 8 f(n) + 49n