AB cannot be congruent all by itself. It has to be congruent to something else. AB could be congruent to x by CPCT, presuming you had a standard vertex naming scheme and x is the side of the second triangle that corresponds to side AB in the first triangle. I'm presuming that x, y, and z are the sides of the second triangle rather than the vertices, since it is standard notation to use upper case letters to label vertices and lower case letters to label sides.
John
My calculator said it, I believe it, that settles it