The midpoint of a line segment XZ where X=(Xx,Xy) and Z=(Zx,Zy) is the average of the coordinates:
(½(Xx+Zx),½(Xy+Zy)). X=(5,6) and the midpoint, Y, of XZ=(14,12), so Xx=5 and Xy=6.
14=½(5+Zx) and 12=½(6+Zy),
28=5+Zx, 24=6+Zy, therefore, Zx=23 and Zy=18, making Z(23,18).