To rotate a point \( (x, y) \) 90 degrees counterclockwise about the origin, you can use the following transformation:
\[ (x, y) \rightarrow (-y, x) \]
If you have the coordinates of point \( A \) as \( (x_A, y_A) \), then the coordinates of point \( A' \) after rotation will be:
\[ A' = (-y_A, x_A) \]
If you provide the coordinates of the original point \( A \) (and potentially points \( B \) and \( C \) if needed), I can help you find the coordinates of \( A' \). Please provide the coordinates for point \( A \).