Express $x_{n+1}$ in terms of $n,\bar{x},s^2$.

Consider a set of $$n$$ observations {$$x_1,...x_n$$}, with mean $$\bar{x}$$ and variance $$s^2$$.When a new observation $$x_{n+1}$$ is added to this set,the mean decreases,but the variance remains the same.Express $$x_{n+1}$$ in terms of $$n,\bar{x},s^2$$.

I can see that $$x_{n+1}$$ should lie below $$\bar{x}$$ but I cannot work with the equal variance criteria.