We will transform a generic point P=(x,y) and then apply both transformations to each of the points.
Reflection across the y-axis: the y coordinate stays the same, but the x-coordinate change its sign.
Rotation 90 degrees counterclockwise: Now we have the point after the first transformation (-x,y) and we have to rotate it 90 degrees counterclockwise. We can draw this as:
If A=(-x,y), the x-coordinate becomes the y-coordinate, and the y-coordinate becomes the x-coordinate, but of the opposite sign. Then, A'=(-y,-x).
Then, for each point (x,y) when both transformation applied we get:
We have the points D, E and F: