I assume that we are dealing with a right angled triangle and that the number you want to find is the base of the smaller triangle. If so, we use Pythagoras' theorem.
You called the unknown ?, so i'll continue using that.
?^2 = 15^2 - 9^2
=> ? = sqrt(15^2 - 9^2)
= 12 m
Just in case what you really wanted was the base of the larger triangle (since the base of the smaller triangle was so easily found and it would make no sense for them to give us the 10 m).
You would use a similar triangle relationship. That is, the ratio of any two sides of the triangles are equal. For instance, base/hypotenuse for the small triangle = base/hyponenuse for the larger triangle.
So you could say:
12/15 = x/25, where x is the base of the large triangle, and 25m is the length of the hypotenuse of the large triangle.
=> x = 25*12/15 = 20 m