Answer:
a =
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
a² + 9² = 12²
a² + 81 = 144 ( subtract 81 from both sides )
a² = 63 ( take square root of both sides )
a =