The coordinates of the midpoint are the arithmetic means of the respective coordinates of the endpoints.
\((3,1) \\
x_1=3 \\ y_1=1 \\ \\
(-5,9) \\
x_2=-5 \\ y_2=9 \\ \\
\hbox{the x-coordinate of the midpoint:} \\
\frac{x_1+x_2}{2}=\frac{3-5}{2}=\frac{-2}{2}=-1 \\ \\
\hbox{the y-coordinate of the midpoint:} \\
\frac{y_1+y_2}{2}=\frac{1+9}{2}=\frac{10}{2}=5\)
The coordinates of the midpoint are B. (-1,5).
