Answer:
The scale factor is

Explanation:
Lets have a look to the co-ordinates of triangle ABC and A'B'C.
The point of C=
and B=
The point of C=
and A'=
By applying distance formula we can calculate the distance of line segment :

BC =
=
=2
Similarly CA=
=8
Scale factor = K=
=
=4
The image and its preimage lies in opposite sides of center of dilation, C=
therefore the scale factor will be negative.
Our final answer = -4