7.
Since it is a right triangle we can use Pythagorean Theorem to solve.
A^2+B^2=C^2
We are given C and B. We are solving for A, so we can input what we know.
A^2+24^2=25^2
A^2+576=625
A^2=49
A=7
Therefore, the missing side is equal to 7.
