Answer:
9 inches
Step-by-step explanation:
The support forms a right triangle with the shelf. So we can use Pythagorean theorem to find the height of the triangle.
c² = a² + b²
(15 in)² = (12 in)² + b²
225 in² = 144 in² + b²
81 in² = b²
9 in = b
The support attaches to the wall 9 inches below the shelf.
