You can check to see if the numbers satisfy the Pythagorean theorem.
9² +12² ?= 15²
81 + 144 = 225 . . . . yes these segments form a right triangle
_____
You know that segments of lengths 3, 4, 5 form a right triangle. This is the first "Pythagorean triple" that most folks run across. The segments given here are all 3 times those numbers, so will also form a right triangle.
