Hello!
You can use the Pythagorean Theorem to solve this.
![a^(2) + b^(2) = c^(2)](https://img.qammunity.org/2019/formulas/mathematics/college/3byu32yniwf4k67ty0w3p5iut2e0wsjpqf.png)
c is the hypotenuse which is the side opposite of the right angle
a and b are the other sides
Put in the values you know
![a^(2) + 24^(2) = 25^(2)](https://img.qammunity.org/2019/formulas/mathematics/college/fwr02ms3yb7ucd0tu8mb1bi8pbx9cdc1tk.png)
Square the numbers
![a^(2) + 576 = 625](https://img.qammunity.org/2019/formulas/mathematics/college/42eh5ei983mylns6vhvesx4yrgpci5n9r6.png)
Subtract 576 from both sides
![a^(2) = 49](https://img.qammunity.org/2019/formulas/mathematics/college/y5orw75yvrm1dl6ryy2mts1bxxs3pu2gcs.png)
Take the square root of both sides
a = 7
The answer is 7
Hope this helps!