We can use the Pythagorean Theorem to see if it is a right triangle.
The Pythagorean Theorem is a² + b² = c².
c = the longest side, or the hypotenuse. In this case, the hypotenuse should be 65. 39 can be a, and 52 could be b, or 52 could be a, and 39 could be b. It doesn't matter.