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?