When doing a transformation (specifically rotation and reflection), how do you do arrow notation if the images aren't on a coordinate plane?

Question

anonymous · Answer

What do you mean 'aren't on the coordinate plane'?

anonymous · Answer

They are on a blank plane without coordinates or anything. Just a flag-shape ABCD and its image A'B'C'D. Something like this

anonymous · Answer

If there isn't a coordinate system, then you get to make up your own.

anonymous · Answer

"Identify the transformation. Then use arrow notation to describe the transformation.

anonymous · Answer

It's a $90^\circ $ rotation (counterclockwise). $$
(x,y) \to (-y, x)
$$

anonymous · Answer

Ok thanks!