We want to calculate a_n, that will be the arithmetic mean between a_(n-1) and a_(n+1).
We can calculate it as:
[tex]\begin{gathered} a_n=\frac{a_{n-1}+a_{n+1}}{2} \\ a_n=\frac{4+(-9)}{2} \\ a_n=\frac{-5}{2} \\ a_n=-2.5 \end{gathered}[/tex]
Answer: a_n = -2.5
