Answer:
You can know if the shapes are similar if the are the same type of shape, have proportional side lengths, congruent corresponding angles (the angles are in the same place proportional to the shape and have the same amount of degrees), and equal scale factors.
In number 19, the polygons are similar because they have proportional side lengths and congruent corresponding angles.
Number 20's trapezoid A and B are not similar because the side lengths are not proportional to each other ( 2 is half of 4, 4 is not half of 6)
Hope this helps!
