To solve this, we need to determine which angle is supplementary to $\angle 3$.

Supplementary Angles:

Two angles are supplementary if their sum equals 180 degrees.

Looking at the diagram, $\angle 3$ and $\angle 5$ appear to be adjacent angles forming a straight line. Since they form a straight line, their measures sum to 180°, meaning $\angle 5$ is supplementary to $\angle 3$.

The correct answer is $\angle 5$, which corresponds to $\angle 6$ in the given answer options.

Would you like more details, or have any questions?

Here are some follow-up questions to deepen your understanding:

1. What does it mean for two angles to be adjacent?
2. How can we identify complementary angles in a similar diagram?
3. Can vertical angles ever be supplementary?
4. How would you find angles in a transversal situation with parallel lines?
5. What is the relationship between opposite angles formed by two intersecting lines?

Tip: When solving for supplementary angles, always look for pairs of angles that form a straight line!