The line was reflected about the line y = -x.
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This question is solved using reflection concepts.
There are various kinds of reflections, 90º clockwise about the origin, 90º counterclockwise, among other, and each of them has a rule.
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In this question:
From this, we have the following rule: (x,y) -> (-y,-x)
Checking the rules, it can be said that the line was reflected about the line y = -x.
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