Answer:
2, 6, and 7.
Explanation:
In a right triangle, the hypotenuse (longest side of the triangle) squared is equal to the sum of the other sides squared.
So for the second triangle, add 36 squared to 15 squared. The result is 1521. Compare that to the hypotenuse (39) squared. The second triangle is a right triangle because the hypotenuse squared is also 1521. I did this with each triangle and the 2nd, 6th, and 7th were right triangles.