90° counter clockwise rotation of point 0(x, y) about origin = (-y, x)
Following are the rules for a counter clockwise rotation of 90 degrees about the origin for a point O(x,y):
1. Invert the sign of the value of y.
2. Switch x and y with each other.
For example, a point A(5,2) after rotation gives a point B(-2,5) using the rules given above. The points on the x-y plane are shown in the image attached.