Answer
$B=(10,13)$.
Work Step by Step
Let $B=(x,y)$.
Since (6,8) is the midpoint of AB,
$(6,8)=(\displaystyle \frac{2+x}{2}, \frac{3+y}{2})$
Equating the x-coordinates,
$6=\displaystyle \frac{2+x}{2}$
$12=2+x$
$x=10$.
Equating the y-coordinates,
$8=\displaystyle \frac{3+y}{2}$
$16=3+y$
$y=13$.
So, $B=(10,13)$.