If you are doing this for geometry (triangles) then both lines are congruent making it a side. Or if it’s asking you what the function is, it’s going to be reflexive property. Depending how many sides or angles your triangle has, it’ll be angle-side-angle, side-side-side, side-angle-side, angle-angle-side, or hypotenuse-leg. To find which corner makes an angle, they will tell you in the given that angle acb (this is an example) makes an angle. To find it, start with the first letter to the last in order, the middle letter will be your angle.