Since we have a right triangle, we can apply Pytagorean theorem, that is,
[tex]x^2+8^2=13^2[/tex]
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
[tex]x^2+64=169[/tex]
Now, by subtracting 64 to both sides, we get
[tex]x^2=105[/tex]
and by taking square root to both sides, we obtain
[tex]x=\sqrt[]{105}[/tex]
Therefore, 105 is the number that belongs in the green box.
