In the sequence $$ x_{0}, x_{1}, x_{2}, ..., x_{n}$$, each term from $$ x_{1}$$ to $$x_{k}$$ is 3 greater than the previous term, and each term from $$x_{k+1}$$ to $$x_{n}$$ is 3 less than the previous term, where n and k are positive integers and k < n. If $$ x_{0} = x_{n} = 0$$ and if $$ x_{k} = 15$$, what is the value of n?