The coordinate of the midpoint M(4,1).
The coordinates of A is (2,8).
The coordinates of B can be determined from the mid-point theorem as,
[tex]\begin{gathered} x=\frac{x_1+x_2}{2} \\ 4=\frac{2+x_2}{2} \\ x_2=8-2 \\ x_2=6 \\ y=\frac{y_1+y_2}{2} \\ 1=\frac{8+y_2}{2} \\ y_2=2-8 \\ y_2=-6 \end{gathered}[/tex]Thus, the required coordinate of the B is (6,-6),.
