Answer:
it's correct because even though rotating 90° is not the same as reflecting about the y-axis, when you reflect again about the x-axis, it becomes the same as reflecting an additional 90°.
so reflecting about y and then about x is the same as rotating 180°.
Step-by-step explanation:
if you reflect (x, y) in 180° it becomes (-x,-y)
when you reflect about the y-axis, (x,y) becomes (-x,y).
when you reflect again about the x-axis, (-x,y) becomes (-x,-y) which is the same as the 180°