To solve this type of problem, you would need to do the reverse of Pythagorean Theorem. C represents the hypotenuse, while a and b represent side lengths.
c^2-b^2=a^2
15^2-12^2=a^2
81=a^2
sqrt 81=sqrt a^2
9=a
Using that formula, we can see that the other side length is 9 feet.
