Identify the hypotenuse of a trinangle by fiving its letter

I have no idea what you mean by "fiving its letter". Only right angle triangles have a hypotenuse. It is the side opposite the right angle, and is the longest side.

To identify the hypotenuse of a right triangle, you don't need to assign it a specific letter. Traditionally, the hypotenuse of a right triangle is labeled with the letter "c". The other two sides of the right triangle are typically labeled as "a" and "b". The hypotenuse is always the side opposite the right angle.