To do this you would have to use the Pythagorean Theorem, which is
a^2+b^2=c^2
The longest side would be your c^2, the smallest side is your a^2, and the remaining side is your b^2
The 7 is your a^2, 24 is b^2, and x is c^2
Plug in the numbers into your equation
7^2+24^2=x^2
Then, just solve:
49+576=x^2
625=x^2
To get rid of the ^2 in x^2, you would have to square root both sides to get x by itself
25=x
So x is equal to 25
Hope this helps!