Answer is GH = 12
The question mentions that there is a perpendicular bisector, and a perpendicular line means that there is a right angle (or more) on that line.
Both triangles are the same because they lie within the same perpendicular bisector, have the same angle and share a common side (JH).
The triangles are the same, so side HI = side GH, so side GH is 12.
