Final answer:
To represent Sam's car as a function of time, a linear equation like f(x) = 2x would indicate constant motion away from the start point. Equations like f(x) = 15 - x or f(x) = 20 - 2x suggest motion toward the starting line or deceleration. The equation f(x) = x suggests a constant speed of 1 unit per second.
Step-by-step explanation:
The equation that represents Sam's car as a function of time is looking for a relationship between the distance from the starting line, f(x), and the number of seconds, x. Without additional information on whether Sam's car is accelerating or moving at a constant speed, we cannot definitively choose an equation. However, if we assume the car is moving at a constant speed without changing direction (which is what these kinds of equations generally represent), then a linear equation like f(x) = 2x would indicate the car travels 2 units of distance for every second that passes. On the other hand, equations like f(x) = 15 - x or f(x) = 20 - 2x typically represent a car moving towards the starting point or decelerating, which might not be what we are looking for in this context. Lastly, f(x) = x would mean the car travels 1 unit of distance per second, which is also a possible scenario if the car were moving at a constant velocity. Without knowing the specifics of Sam's car's movement, we cannot conclusively select the correct equation.