Question 934328
This same question identically was asked two more times.  This response will not be a formal proof, but just a helpful description.


You know that AB contains the collinear points D and E, and that AB is a segment, forming supplementary angles at D and at E.  You know from this that angles ADC and BEC are congruent,... something about supplementary angles of congruent angles...


You have at least, these ... statements:


AD congru BE  |  Given
angles CDE congru CED    |  Given
angles ADC congru BEC    |  Reason discussed above
Triangles ADC and BEC congruent   |   Not know how to state this theorem, but two sides and included angles....
Segments AC congru  BC    |   Corresponding parts of congruent triangles...



Excuse abbreviations.