Answer:
4/9
Step-by-step explanation:
The easiest segment to work with is BC, since the y-values are the same for coordinates B and C and for B' and C'.
The length of BC = 9 -(-9) = 18.
The length of B'C' = 7 -(-1) = 8.
The scale factor is B'C'/BC = 8/18 = 4/9.