A is the mid point ==> AL = AB = 2AB
6x-17=2(2x+3) => 6x -17 = 4x +6 => 2x = 23 =>x=23/2
then just plug x into LB and AB
I hope you can read this, and I hope you understand how I did 7. I probably overworked it, and it was possibly a simpler way to get it.
It will be easier to explain how to solve this question in a square. If you rotate 4-sided square from the centre, you need to rotate it 90 degrees. The formula for this would be: 360 ° /the side count. In a square, it would be 360° /4= 90°.
In 18-sided polygon, the calculation would be: 360°/ 18 side= 20°