Answer:
36 miles
Step-by-step explanation:
(Imagine a huge triangle on a perfectly open space-- not with streets or houses or cornfields.)
The dimensions of the two sides given are multiples of a classic Pythagorean Triple: 3, 4, 5.
9 is 3x3, 12 is 3x4 so the remaining side would be 3x5 15
Add those 3 sides to get the total distance, 36 miles.