This video explains what the transformation matrix is to rotate 90 degrees anticlockwise (or 270 degrees clockwise) about the origin.
Lecture 7 Geometric Transformations (Continued) - ppt download
Matrix Rotations
90 Degree Anticlockwise Rotation Transformation Matrix
Computer Graphics - 3D Rotation Transformations - GeeksforGeeks
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How to rotate 90 degrees clockwise about a point, not the origin, say point A (-2, 3) - Quora
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Do you know matrix transformations
Matrix Rotations