First, you know that B is the mid point, meaning AB is equal to BC. Therefore, you can set up the equation: x+5=2x-11. Then, you solve for x. Once you are done solving, you should get 16. Hope this helps!
There are 256 possible combinations of fireworks, if they can be repeated in each 4 firework combinations. If they cannot be repeated, there are 24 possible combinations.