5. The first hexagon has six sides, the second one borrows 1 side of the first hexagon, the third one borrows 1 side of the second hexagon, and so on, 6+5+5+5.

If you are only a sophomore, you can basically understand it like this. If you are a senior, you can use three-dimensional graphics. First, you can use 12 fire sticks to build the upper and lower hexagons. The third hexagon uses two sides corresponding to the upper and lower hexagons, and the fourth hexagon uses two opposite sides, so that the third and fourth hexagons only use four matchsticks, 6.