Since we have a right triangle, we can apply Pytagorean theorem, that is,
where the 13 units side is the hypotenuse. Remember that the hypotenuse is always in front of the right angle.
From our last equation, we have
Now, by subtracting 64 to both sides, we get
and by taking square root to both sides, we obtain
Therefore, 105 is the number that belongs in the green box.