Answer:
b = 9
Step-by-step explanation:
Using Pythagoras' identity in the right triangle.
The square on the hypotenuse is equal to the sum of the squares on the other 2 sides, that is
b² + 12² = 15²
b² + 144 = 225 ( subtract 144 from both sides )
b² = 81 ( take the square root of both sides )
b = = 9