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Answer:
(x, y) ⇒ (y, -x)
Explanation:
Rotation 90° CW is described by the transformation ...
(x, y) ⇒ (y, -x)
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Additional comment
If you consider a point on the x-axis after a 90° CW rotation, you realize it must have been on the y-axis before the rotation. That is, the new x-value is the old y-value.
Similarly, a point on the y-axis after rotation 90° CW must have been on the -x axis before the rotation. The new y-value is the old -x value:
(x, y) ⇒ (y, -x) . . . . . (old coordinates) ⇒ (new coordinates), 90° CW