For this case we have by definition that, a flat angle is the space included in an intersection between two straight lines whose opening measures 180 degrees. So, the angle between A and B is 180 degrees.
If we are told that the ACD angle is 100 degrees, then the DCB angle should be 80 degrees.
The angle ACE is opposite by vertex to the angle DCB, then it is also 80 degrees; since by definition, two angles opposed by the vertex are congruent or equal.
