Graphing transformations - Word problem

Question

A car decelerates at a constant rate as it approaches a red light. The velocity of the car in meters per second as it decelerates is modeled by the function f(x)=−4x+20, where x is time in seconds. After five seconds, the car reaches a complete stop. Which situation is appropriately modeled by reflecting f(x) across the x-axis?

1. The car immediately begins to accelerate at twice the same rate it had previously decelerated.

2. The car begins to drive in reverse.

3. The car immediately begins to accelerate at the same rate it had previously decelerated.

4. The car remains stopped.

Answer 1

You want to draw this graph in order to picture what's going on.  Following the description, the original graph has the car decelerating at a rate of 4 m/s every second (or 4m/s²).  The slope of -4 tells us this.  When you reflect the graph over the x axis, the slope now becomes 4, BUT the y-intercept (the starting point where time = 0) is now -20.  Negative velocity is typically defined as movement in the opposite direction, so you want to pick answer 2.