The rule for reflecting a point across the y-axis is to change the sign of the x-coordinate while keeping the y-coordinate the same. If a point $(x, y)$ is reflected across the y-axis, it becomes $(-x, y)$.

Therefore, the rule describing the reflection of quadrilateral PRST across the y-axis to create quadrilateral $P'R'S'T'$ is:

$(x, y) \rightarrow (-x, y)$

Would you like more details or have any questions about reflections or transformations?

Here are 5 related questions to explore further:

1. How would the rule change if the quadrilateral were reflected across the x-axis?
2. What would the rule be for reflecting the quadrilateral across the line $y = x$?
3. How can you determine the coordinates of the new vertices after reflection?
4. What is the effect of reflecting a shape twice across two different axes?
5. How does reflection differ from rotation and translation in transformations?

Tip: Always check the orientation of the shape after transformation to ensure it hasn't been distorted.