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