By the converse of the Pythagorean Theorem, if 8.6 ^2 = 8.1 ^2 + 2.6 ^2 , then the triangle is right. 73.96 is not equal to 72.37 so the triangle is not a right triangle.
8.6^2 is greater than (8.1^2 + 2.6^2) which means that the triangle is obtuse. The obtuse angle is opposite the 8.6 side.