This is because it wants you to represent the answer in Slope-Intercept Form. Which is y = mx + b. B is the y intercept, which is where it starts. On a graph, the y-intercept starts at ZERO. The 3 is at the zero, so you know what the first number is. Then, you need to see the fluctuation (change) in the value of each number in the y as you increase in x. As the x increases by one, you can see that the y decreases by two. So now you know, the ROC or Rate of Change is two, which is your slope. Slope is m. So if m is two, that makes 2x.
