1. **State the problem:** We are given a straight road with stop signs at both ends and a fire hydrant at its midpoint. The fire hydrant is at point $(12,7)$ and one stop sign is at $(3,11)$. We need to find the coordinates of the other stop sign. 2. **Formula used:** The midpoint formula for two points $(x_1, y_1)$ and $(x_2, y_2)$ is: $$\text{Midpoint} = \left(\frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2}\right)$$ 3. **Apply the formula:** Let the other stop sign be at $(x, y)$. Given the midpoint $(12,7)$ and one endpoint $(3,11)$, we have: $$\left(\frac{3 + x}{2}, \frac{11 + y}{2}\right) = (12, 7)$$ 4. **Set up equations:** $$\frac{3 + x}{2} = 12$$ $$\frac{11 + y}{2} = 7$$ 5. **Solve for $x$:** Multiply both sides by 2: $$\cancel{2} \times \frac{3 + x}{\cancel{2}} = 12 \times 2$$ $$3 + x = 24$$ Subtract 3 from both sides: $$x = 24 - 3$$ $$x = 21$$ 6. **Solve for $y$:** Multiply both sides by 2: $$\cancel{2} \times \frac{11 + y}{\cancel{2}} = 7 \times 2$$ $$11 + y = 14$$ Subtract 11 from both sides: $$y = 14 - 11$$ $$y = 3$$ 7. **Final answer:** The other stop sign is located at $(21, 3)$, which corresponds to option C.