What do the lines making a circle around the low-pressure area indicate?

Well,

When you see an "L" surrounded by circles, the circles tell you that each consecutive circle which is further away from the "L" has a higher pressure.

The answer is 36. The ruler and tree are measured at the same time, so the scaling is the same. You divide 12 by 8, which is 1.5. Since the height of the pine tree is what you are trying to find, you must multiply 24(the length of the shadow) times 1.5, which is how you get 36. :)