EXPLANATION
Let's call x to the missing side:
Applying the Pythagorean Theorem give us the missing side as shown as follows:

Replacing terms:

Solving the powers:

Subtracting 225 to both sides:

Subtracting numbers:

Applying the square root to both sudes:
![\sqrt[]{400}=x](https://img.qammunity.org/2023/formulas/mathematics/college/tzmma0kwdtrc9skdhvbprls57pr2xyn9hj.png)
Switching sides:
![x=\sqrt[]{400}](https://img.qammunity.org/2023/formulas/mathematics/college/h9e5i571u2zxba8sdy8yple4u9wqsq6wrd.png)
Simplifying:

The answer is x=20