Please answer this one, it's quite hard! :/

Answer:
3.5
Step-by-step explanation:
8.5 are the long sides
8.5+8.5=17
24-17=7
so both short sides add to 7
7 divided by 2 = 3.5
Answer:
Hey!
Your answer is 3.5cm!
Step-by-step explanation:
We are given one side's length... 8.5cm
As there are 2 sides we add (or multiply by 2) these sides together...
8.5 + 8.5 = 17
We are missing two shorter sides now...
So we have to divide 17 by 2 to get...
17 / 2 = 3.5! *this is your answer!*