1. **Stating the problem:** Given the morning temperature is $-4^\circ C$ and it increases by $9^\circ C$ by noon, find the temperature at 15:00 given by $2x - 6$ where $x$ is the noon temperature. 2. **Find the noon temperature $x$:** $$x = -4 + 9 = 5$$ 3. **Calculate temperature at 15:00:** $$2x - 6 = 2(5) - 6 = 10 - 6 = 4$$ 4. **Temperature decreases by $1^\circ C$ every 2 hours.** From 15:00 to 21:00 is 6 hours, so decrease is: $$\frac{6}{2} \times 1 = 3$$ 5. **Calculate temperature at 21:00:** $$4 - 3 = 1$$ 6. **Answer:** The temperature at 21:00 is $1^\circ C$. **Final answer:** d. 1°C