HELP!! im using the last of my points for this, im really bad at math and im stressed trying to do this.

Answer:
D. 19.6in
Step-by-step explanation:
ΔAFC is similar o ΔA'FC'. Therefore, their sides are in ration to one another:
FA/F'A'=AC/A'C'
9/(9+12)=8.4/A'C'
9/21=8.4/A'C'
3/7=8.4/A'C'
A'C'=8.4/(3/7)=19.6