ANSWER IS 24..
You would set it up in a^2+b^2=c^2 form. Since it said your hypotenuse is 25, that would be your c^2 and 7 would be your a^2 or b^2. it dont matter. but anyway 7^2 + b^2 = 25^2. square your numbers and youll get 49+ b^2=625. subtract 49 from 625 and you get 576. take a square root of that and you get 24.