Chapter 1 - Tools of Geometry - 1-7 Midpoint and Distance in the Coordinate Plane - Practice and Problem-Solving Exercises - Page 54: 20

$S(9,-28)$

Use the midpoint formula to find the x-coordinate. $M=\frac{x_1+x_2}{2}$ $5=\frac{x_1+1}{2}$ $5\times2=\frac{x_1+1}{2}\times2$ $10=x_1+1$ $10-1=x_1+1-1$ $9=x_1$ Use the midpoint formula to find the y-coordinate. $M=\frac{y_1+y_2}{2}$ $-8=\frac{y_1+12}{2}$ $-8\times2=\frac{y_1+12}{2}\times2$ $-16=y_1+12$ $-16-12=y_1+12-12$ $-28=y_1$