To know the scale factor, you must take the ratio of the same sides in each of the figures. For example, here in these figures, the length of the vertical lines is given in each of the figures so we'll use them to calculate the scale factor A to B. Note: you should put A' value on top because it is asking you about the scale factor from A to B and not B to A.
Therefore, by simply diving the lengths of these sides, we get:
15/12 = 5/4 = 1.25
1.25 is the scale factor for A to B.