We have the following:
We must pass the time from 19 years to periods of 6 years, as follows
[tex]\frac{19}{6}=3.17[/tex]That is, 19 years are 3.17 periods of 6 years
now, the formula to calculate the value of the car is
[tex]c=A\cdot(1-x)^n[/tex]Where A is the initial value, x is the depreciation and n is the time in periods of 6 years
Replacing:
[tex]c=18000\cdot(1-0.5)^{3.17}=1999.89[/tex]Therefore the value of the car would be approximately 1999.9 dollars