If the third side is the hypotenuse, then (using the Pythagorean Theorem) the square of its length is the sum of the squares of the other two sides: 132+52 , so the length of the hypotenuse is √194=13.92838827718412...
However, it is also possible to have a right triangle with a hypotenuse of 13 cm and one side of 5 cm, in which case the third side would be: √132−542=12.
Of course, the second answer may seem “nicer” but both are correct and the question is ambiguous.