Answer:
P'(-9, 3)
Step-by-step explanation:
The transformation for rotation counterclockwise about the origin by some angle α will be ...
(x, y) ⇒ (x·cos(α) -y·sin(α), x·sin(α) +y·cos(α))
When the angle is α = 90°, this reduces to ...
(x, y) ⇒ (-y, x)
You have (x, y) = (3, 9), so the image point is P'(-9, 3).
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Additional comment
The "work" is applying the right sign and choosing the right coordinate to fill in the values (-y, x). I find it easier to look up the transformations for ±90° and 180°, rather than try to derive them from first principles every time. Your text may have a list:
- 90° CCW or 270° CW: (x, y) ⇒ (-y, x)
- 90° CW or 270° CCW: (x, y) ⇒ (y, -x)
- 180°: (x, y) ⇒ (-x, -y)
