Hi there!
The included angle of sides JG and GH is angle G (JGH). This is because that is the angle the two line segments create.
Angle H is the opposite angle to side JG. If you draw a line from the angle straight down and extended the base, you'd see that the line intersected that side and that side only.
The side opposite to angle J is side GH. As I explained above, drawing a line would show you the intersection.
Hope this helps!! :)
If there's anything else that I can help you with, please let me know!