Explanation
One of the properties preserved under a dilation from the pre-image to the image is that the angle measures remain the same. In other words, the pre-image and image are similar. When two figures are similar, their corresponding angles are equal.
Thus, if the measure of angle ABC is 21, the measure of angle A'B'C' is also 21.
Answer
The measure of angle A'B'C' is 21.
