Answer:
The dilation scale factor is 
. 
Explanation:
The image is the dilated form of its preimage if and only if the following conditions are observed:
1) 

2) 

3) 

4) 

5) 

If we know that 
, 
, 
, 
, 
, 
, 
 and 
, then the coefficients are, respectively:
, 
, 
, 

As 
, we conclude that the dilation scale factor applied in the preimage is equal to 
.