**Following number line is representing the temperature:**

The temperature decreases 2^{o}C = 1 hour

The temperature decreases 18^{o}C = 1/2 x 18 =9 hours

Total time = 12 noon + 9 hours = 21 hours = 9 pm Thus, at 9 pm the temperature would be 8^{o}C below 0^{o}C.