To find the coordinates of vertex \( X' \) after reflecting triangle \( XYZ \) over the line \( y = 1 \), we start with the coordinates of vertex \( X \), which are \( (-3, 8) \).
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Determine the distance from vertex \( X \) to the line of reflection \( y = 1 \):
- The y-coordinate of \( X \) is \( 8 \).
- The distance from \( 8 \) to \( 1 \) is \( 8 - 1 = 7 \) units.
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Since reflection over the line means moving the same distance to the opposite side:
- We subtract the distance from \( 1 \) (the line of reflection):
- New y-coordinate after reflection: \( 1 - 7 = -6 \).
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The x-coordinate remains the same, so:
- The new coordinates of vertex \( X' \) are \( (-3, -6) \).
Thus, the coordinates of vertex \( X' \) are \( \boxed{(-3, -6)} \).