This video explains what the transformation matrix is to rotate 90 degrees anticlockwise (or 270 degrees clockwise) about the origin.
matrices - How is that a rotation by an angle θ about the origin can be represented by this transformation matrix? - Mathematics Stack Exchange
Rotation matrix - Wikipedia
180 Degree Rotation Transformation Matrix
Lecture 7 Geometric Transformations (Continued) - ppt download
Rotation Transformation Matrix
Understanding Geometric Transformation: Rotation using OpenCV-Python
Linear transformation examples: Rotations in R2 (video)
Math Alive Geometry 1
Geometry Rotations Clockwise and Counterclockwise Explained! — Mashup Math
Complete Guide On 2D Array (Matrix) Rotations - Data Structure and Algorithms Tutorial - GeeksforGeeks