Hello!
You can use the Pythagorean Theorem to solve this.
[tex] a^{2} + b^{2} = c^{2} [/tex]
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
[tex] a^{2} + 24^{2} = 25^{2} [/tex]
Square the numbers
[tex] a^{2} + 576 = 625[/tex]
Subtract 576 from both sides
[tex]a^{2} = 49[/tex]
Take the square root of both sides
a = 7
The answer is 7
Hope this helps!