Answer:
12
Explanation:
Pythagorean Theorem states that a^2 + b^2 = c^2. Using the numbers that you're given you should have an equation of a^2 + 9^2 = 15^2.
Once you square your terms you should have a^2 + 81 = 225
You can find a^2 by subracting b^2 from c^2. So 225 - 81 = 144
Then you find the square root of 144, which is 12.