Answer: 15 units
Explanation:
 Since this is a right triangle, we will use the Pythagorean theorem. a and b are the legs, c id the hypotenuse. Here, we need to solve for the hypotenuse.
 a² + b² = c²
 9² + 12² = c²
 81 + 144 = c²
 225 = c²
 
 = c
 15 = c
 15 units.