Dilation preserves the parallel lines, it means that the image line C'D' of the line CD are parallel each to other and their slopes are the same. So you have only to find the slope of CD.
If line is passing through the points
and
, the slope is

From the diagram you can find coordinates of points C and D:
C(5,4), D(4,2).
Thus, the slope of CD is

Answer: 2.