\[\boxed{\mathbf{85.}}\]
\[\overline{x} = \frac{S_{n}}{n}\]
\[S_{n} = \overline{x} \cdot n.\]
\[S_{2} = S_{n} + a = \overline{x} \cdot n + a;\]
\[\overline{x_{2}} = \frac{S_{n} + a}{n + 1} = \frac{\overline{x} \cdot n + a}{n + 1}.\]
\[Ответ:\ \frac{\overline{x} \cdot n + a}{n + 1}\text{.\ }\]