Since the figure was rotated, the image figure has the same corresponding sides, the thing that changed was the position of the points and the figure but not the length of the sides.
First, we identify corresponding sides (equal sides). This is shown in the following image by drawing corresponding sides in the same color:
As we can see, the length of side AB is equal to the length of side A'B', because they are corresponding sides.
Anwer:
[tex]\text{Length AB=Length A'B'}[/tex]
