When you rotate 270 clockwise, that's the same as 90 anticlockwise. (90, 180, 270, 360)
Look at the point (2,1) I've boxed it into a rectangle and then showed the whole rectangle rotating.
That corner moves to (-1,2) For 90 degrees that sort of thing always happens. The x and y coordinates swap over and one becomes negative.
It happens because the across distance in the rectangle becomes the up distance and the up becomes the across (but now going to the left ... ie. it is now a negative amount).
2 ----> -1
1 ----> 2
So you should be able to do the other points the same way.