1. **Problem Statement:** Given that $\angle 11 \cong \angle 2$, determine which lines are parallel and state the postulate or theorem that justifies your answer. 2. **Identify the angles and lines:** - $\angle 11$ and $\angle 2$ are congruent. - Lines involved are $a$, $b$, $c$, and $d$. 3. **Recall relevant postulates/theorems:** - Corresponding Angles Postulate Converse: If corresponding angles are congruent, then the lines are parallel. - Alternate Interior Angles Theorem Converse: If alternate interior angles are congruent, then the lines are parallel. 4. **Analyze the given angles:** - $\angle 11$ and $\angle 2$ are positioned such that they are alternate interior angles formed by lines $c$ and $d$. 5. **Conclusion:** - Since $\angle 11 \cong \angle 2$ and they are alternate interior angles, by the Alternate Interior Angles Theorem Converse, lines $c$ and $d$ are parallel. **Final answer:** $$ c \parallel d \quad \text{by the Alternate Interior Angles Theorem Converse} $$