Like that ??? Am right ??

That really does seem correct lol I think the variable is right since 5 represents 3x so 9 +3(5) =24

The exponential formula for this situtation will be given by:
f(x)=ae^(-bx)
where:
a=initial price
b=rate
x=time in years
hence the formula will be:
f(x)=28750e^(-0.12x)
thus the value of the car in 2018 will be:
x=2018-2012=6 years
thus
f(6)=28750e^(-0.12*6)
f(6)=$13,994.13