To determine the hypotenuse of the triangle.
The hypotenuse is the opposite side of the right angle in the triangle. It’s also the longest side of the triangle.
If a problem says that to calculate the length of hypotenuse c in a triangle with side a, side b, and hypotenuse c.
Then you are working with a right-angled triangle.
Then by Pythagoras theorem, determine the hypotenuse.
Then the hypotenuse side is calculated.