Question

If quadrilateral A B C D rotates 90° counterclockwise about the origin, what are the coordinates of A′ in quadrilateral A′B′C′D′ ?

Answer 1

If this is all the information you have I can explain it like this.

Your original coordinates are written in the form (x,y).
If you rotate the quadrilateral about the origin 90 degrees, it will be (-y,x).

For example, if the original coordinates for A is (4,6), then A' is (-6,4).