To solve this question, knowing that this is a right triangle since the differences in side lengths are quite large, and thus it cannot be an equilateral triangle, assuming it is a right angle triangle,
Use Pythagorean theorem to solve for the hypotenuse or the length of the diagonal.
C^2 = 81 + 1600
C^2 = 1681
C = 41 units.
The longest side of the triangle is 41 units.