Answer:
23
Step-by-step explanation:

In 7 years, the car was 18,000 and after 5 years it is worth 10,500 so if you subtract 18000-10500=7500 and if you do 7500/5=1500 so this means that the car was going down 1500 in value every year.
The answer will be x = 8.
I hope this helps you! :)