Answer
$B=(5,-5)$
Work Step by Step
The midpoint $M=(x,y)$ of the line segment from $P_{1}=(x_{1},y_{1})$ to $P_{2}=(x_{2},y_{2})$ is
$M=(x,y)=(\displaystyle \frac{x_{1}+x_{2}}{2},\frac{y_{1}+y_{2}}{2})$
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Given $A=(-1,8),B=(x,y),M=(2,3)$
$ \begin{aligned}
& & & & & & \\
\frac{-1+x}{2} & =2 & /\times 2 & & \frac{8+y}{2} & =3 & /\times 2\\
-1+x & =4 & /+1 & & 8+y & =3 & /-8\\
x & =5 & & & y & =-5 &
\end{aligned}$
$B=(5,-5)$
