#### Answer

1. Given
2. Definition
3. Substitution
4. Substraction Property of Equality
5. Definition

#### Work Step by Step

1. $\angle 1$ is supplementary to $\angle 2$
$\angle 3$ is supplementary to $\angle 2$
2. $m\angle1 +m\angle2 =180^{\circ}$
$m\angle3 +m\angle2 =180^{\circ}$
3. $m\angle 1 +m\angle2 =m\angle3 + m\angle 2$
4. $m\angle 1=m\angle3$ ( By subtracting $m\angle 2$ from both sides)
5. If two angles have the same angle measure, they are congruent.
Therefore $\angle 1 \cong \angle 3$