I'm not sure how to draw it and show you, but I will type it out and hopefully you will understand.
4-nonene means the double bond is the fourth possible one:
CH 3 - CH 2 - CH 2 - CH=CH-CH 2 - CH 2 - CH 2 - CH 3
adding F2 means F2 are added across the double bond
-> CH 3 - CH 2 - CH 2 - CHF-CHF-CHF- CH 2 - CH 2 - CH 2 - CH 3
So: C9H18+F2 --> C9H18F2
