\[\boxed{\mathbf{564.}\mathbf{\ }}\]
\[\frac{7,8 + x}{2} = 7,2\ \ \ \ \ \ \]
\[7,8 + x = 7,2 \cdot 2\]
\[7,8 + x = 14,4\]
\[x = 14,4 - 7,8\]
\[x = 6,6.\]
\[Ответ:\ \ 6,6.\]