For this problem, we could use the pythagorean theorem as follows:
The pythagorean theorem states that the square of the hypotenuse equals to the sum of the square of each one of the sides of the triangle:

Finally,
![x=\sqrt[]{245}](https://img.qammunity.org/2023/formulas/mathematics/college/y6e8s5fqtgckfpuhcisx7w1f1gj563lm73.png)
So, the number that you must put in the green box is 245.