Answer
See the picture below:
Work Step by Step
Given: $\angle 1$ and $\angle 2$ are supplementary. ($\angle 1+ \angle 2=180^{\circ}$)
$\angle 1$ and $\angle 3$ are supplementary. ($\angle 1+ \angle 3=180^{\circ}$)
Prove: $\angle 2\cong \angle 3$
