It will take 18 hours
Explanation:
If it needs 9 windows and started at 6 then we can use 9-6 and we get 3 more windows that need installation. If it take 6 hours and 3 more windows need to be installed then multiply 6 by 3 and you get 18.
If you have to write it as an equation it would be y=1/6x+6
Brainliest my answer if this helps?