Answer:
15
Step-by-step explanation:
Without an angle, finding the third side is impossible
However, I'm assuming this a right tirangle
Using the Pythagorean Theorem, a^2+b^2=c^2 (only for right triangles, however Law of Cosines works for all triangles) where c is the hypotenuse...
9^2+12^2=c^2
81+144=c^2
225+c^2
Square root both sides
c=15
