Answer with Step-by-step explanation:
In a right angled triangle the longest side is its hypotenuse.
And according to Pythagorean theorem:
Hypotenuse is equal to square root of the sum of squares of the legs of the right angled triangle.
i.e.

where x and y are legs of right angled triangle
Here, x=13 and y=17


Hypotenuse= 21.4 cm
Hence, measure for the longest side of the triangle is:
21.4 cm