3.1: Perpendicular bisector of $AB$; point $D$

The line $AB$ is such that the points $A$ and $B$ have coordinates $(-4, 6)$ and $(2, 14)$ respectively.

The point $C$, with coordinates $(7, a)$, lies on the perpendicular bisector of $AB$. Find the value of $a$.

Given that the point $D$ also lies on the perpendicular bisector of $AB$, find the coordinates of $D$ such that the line $AB$ bisects the line $CD$.