#### Answer

1. Given
2. Definition
3. Substitution
4. Definition

#### Work Step by Step

1. Both $\angle 1$ and $\angle 2$ are right angles.
2. By definition $m\angle 1 = 90^{\circ}$, $m\angle 2 =90^{\circ}$
3. $m\angle1=m\angle2$
4. If two angles have the same angle measure, they are congruent.
Therefore, $\angle1\cong\angle2$.