Answer:
Hello! The transformation used here is reflection. A reflection is a type of rigid motion transformation that creates a mirror image of the pre-image across a line of reflection. (in this case, the y axis) Which basically means the shape is flipped across an axis. Both the original and the reflected image are congruent. Now, to prove that the reflection was done correctly, count the distance between each point on the figure and the line of reflection (count the distance between each point of the shape and the y axis, the distance should be the same when you count the distance of the axis and the reflected image as well). Hope this helps you