Since this is an isosceles triangle <H and <J would have to equal the same amount which means that x would have to equal the same thing. Setting this up would look like 6x + 18 = 8x.
First you'd subtract 6x from both sides. (What you do to one side must be done to the other)
Then you'd be left with 18 = 2x so you would divide by 2 to get x. once you get x you have to plug the number into each equation. Since x = 9 both of the angles would equal 72.
Lastly you would have to add the two angles together and divide it by 180 since the total of the angles of all triangles equals 180.
180 - 144 = 36. (I hope that makes sense)
So <G = 36°
