ANSWER
A'(1, -1), B'(6, 3), C'(6, -1)
Step-by-step explanation
We have triangle ABC, and we have to rotate it about vertex A 90 degrees counterclockwise,
Usually, when the center of rotation is the origin, the coordinates of each point exchange and the x-coordinate becomes negative (x, y) → (-y, x). But in this case, the center of rotation is A(1, -1).
If we translate the origin to point A, the new coordinates of the points would be,
Rotating these points about the new origin, the coordinates on the red coordinate plane are,

Let's graph these points,
Now, these points, in the original coordinate plane, have the coordinates,
Hence, the coordinates of the translated vertices are A'(1, -1), B'(6, 3), and C'(6, -1).