All you had to do was add 9 + 6 because the 3 to triangle by eah are worth 3 and so are the bottom ones, to that makes 9 + 6 so just add and get 15 if that makes sence, i had the same question.
There needs to be more information. We at least need to know what one of the side measurements for each triangle are. Only then can we give you the answer.
