Assume the next disposition:
1 ft is equivalent to 0.3048 meters, then 12 ft is equivalent to 12*0.3048 = 3.6576 m
If the sides make a right triangle, the Pythagorean theorem must be satisfied.
3.6576² + 9² = 15²
13.378 + 81 = 225
94.378 ≠ 225
Then, the three sides of your garden don't create a right triangle