First, the question is probably what this function is big O of. Second, there are infinitely many answers, and you are probably looking for the slowest-growing one. Then 6^n is correct.
The function log(log(n)) grows very, v e r y slowly. If the logs are based 10, it returns 2 for n=10¹⁰⁰, which is greater than the number of particles in the universe. Nevertheless, this function tends to infinity as n → ∞.
I am not sure what exactly you difficulty with log(log(n)) is. Feel free to post concrete questions about these functions.