Answer
a.) $ y = -3000x + 24000$
b.) After 5 years the car's value will be $9000
c.) See attached image. The blue point is the solution in part b.)
Work Step by Step
$a.)\ m = \dfrac{y_{2} - y_{1}}{x_{2} - x_{1}} \longrightarrow -3000 = \dfrac{y - 24000}{x - 0} \longrightarrow -3000 = \dfrac{y-24000}{x} \longrightarrow -3000x = y -24000 \longrightarrow y = -3000x+24000$
$b.)\ 9000 = -3000x + 24000 \longrightarrow -3000x = 9000 - 24000 \longrightarrow x = \dfrac{-15000}{-3000} \longrightarrow x = 5$
After 5 years the car's value will be $9000