W is in the lower left corner located at (-1,-2)
W' is in the upper left corner located at (-2,1)
Notice that the original Y value (-2) became the new X value (-2) and the original X value (-1) is the inverse of the new Y value (1).
If you look at the coordinates of the other two pints you would see it was the same transformation.
When the X and Y coordinates switch like this, it is a 90 degree clockwise rotation around the origin.