we assume E, G and X lie alphabetically along the line, so if E has a coordinate of 3 and EX=12, the coordinate of X would be 12+3=15

the midpoint of EX is then (15-3)/2 = 6 away from E, or at 3+6=9

EG=6, however, the point G lies at '3+6=9'

therefore point G has a coordinate of 9 and lies at the midpoint of EX, if our assumptions about the problem are correct