1. The problem asks to find the equation representing the cost $c$ of using a carpooling service for $d$ days based on the graph. 2. From the graph, the cost starts at $0$ when $d=0$ and increases linearly, passing through points such as $(1,6)$, $(2,12)$, $(3,18)$, etc. 3. This indicates a linear relationship of the form $$c = md + b$$ where $m$ is the slope and $b$ is the y-intercept. 4. Since the line passes through the origin, $b=0$. 5. The slope $m$ is the change in cost divided by the change in days: $$m = \frac{6 - 0}{1 - 0} = 6$$ 6. Therefore, the equation is $$c = 6d$$ which means the cost increases by 6 dollars per day. 7. Among the options, the correct equation is $$c = 6d$$.