Short Answer 
Alternate exterior angles are pairs of angles located on opposite sides of a transversal that intersects two parallel lines, and they are congruent. To determine these angles, identify the transversal and look for angles outside the parallel lines; for example, angle 13 and angle 5 are alternate exterior angles relative to angle 11.
Step 1: Understand the Definition
To identify alternate exterior angles, it is crucial to first understand their definition. Alternate exterior angles are the pairs of angles that lie on opposite sides of a transversal intersecting two parallel lines. These angles are found outside the parallel lines and are congruent.
Step 2: Identify the Parallel Lines and Transversal
In the given situation, we have two sets of parallel lines: r is parallel to s, and t is parallel to u. By identifying the transversal that intersects these parallel lines, we can determine which angles are alternate exterior angles. Look for angles that are on opposite sides of this transversal.
Step 3: Locate the Alternate Exterior Angles
Based on the figure provided, we can identify the alternate exterior angles related to angle 11. Specifically, angle 13 and angle 5 are located on the exterior sides of the parallel lines and on opposite sides of the transversal. Therefore, these angles confirm the pairs of alternate exterior angles formed in this scenario.