Midpoint formula: [tex](m_x, m_y) = (\frac{x_1+x_2}{2}, \frac{y_1+y_2}{2})[/tex]
Substitute in your given midpoint and point s to solve for r:
[tex](9,8)=(\frac{10+x_r}{2}, \frac{10+y_r}{2})[/tex]
[tex]9=\frac{10+x_r}{2}[/tex]
[tex]18 = 10+x_r[/tex]
[tex]x_r = 8[/tex]
[tex]8=\frac{10+y_r}{2}[/tex]
[tex]16 = 10+y_r[/tex]
[tex]y_r = 6[/tex]
[tex]\textbf{r = (8,6)}[/tex]