\[C(x;y):\]
\[x = \frac{- 3 + 9}{2} = \frac{6}{2} = 3;\]
\[y = \frac{3 + 7}{2} = \frac{10}{2} = 5.\]
\[C(3;5).\]
\[Ответ:C(3;5).\]