1. **State the problem:** We have a proportional relationship between the number of oatmeal raisin cookies Mia buys, $x$, and their total cost in dollars, $y$. 2. **Identify the relationship:** Since the graph is a straight line passing through the origin $(0,0)$ and the point $(10,10)$, the relationship is linear and proportional. 3. **Write the formula:** The general formula for a proportional relationship is: $$y = kx$$ where $k$ is the constant of proportionality (slope). 4. **Find the constant of proportionality:** Using the point $(10,10)$: $$k = \frac{y}{x} = \frac{10}{10} = 1$$ 5. **Write the equation:** $$y = 1 \times x = x$$ 6. **Interpretation:** This means the cost in dollars is equal to the number of cookies bought. **Final answer:** $$y = x$$