Answer:
When rotating a rectangle 270° counter clockwise, you change the vertices in this way: (x,y) ---> (y,-x).
In other words, the "x" and "y" values switch, and the "x" becomes negative. If the "x" value is already negative, it becomes positive.
E' = (-8,6) ----> (6,8)
F' = (-8, 10) ----> (10,8)
G' = (-3, 10) ----> (10,3)
H' = (-3, 6) ----> (6,3)