See this.
Let ABC is not a straight line. then let us produce it back untill it intersects c(o,r) at A'.Join A'D and A'E.
Now <DBE=<EBC
So <DAE=<DBE(Angles in the same segment)
Also <DAE=<DA'E
Here <DAE and <DA'E are corresponding angles. And as they are equal so AE || A'E and that is not possible as both intersects at E.
Hence ABC is a straight line.