To find a set of 3 leg lengths that will form a right-angled triangle, the square of the largest leg must be equal to the sum of the squares of the other 2 legs.
We try out all the options:
39² = 1521, 15² + 36² = 1521.
12² = 144, 8² + 4² = 80.
17² = 289, 13² + 9² = 250.
Since 39² equals 15² + 36², option A is the correct answer.
