$0.42 that is what your answer would be
Using the factor label method, you would set up your ratio like so:


There will be 9 trips the ferry makes and on the last trip there will be 5 cars on the ferry.
The line goes 3 right for every 1 down, so the gradient is -1/3
The y-intercept is +4, so the equation is actually y=-1/3x+4, so Chris is wrong.