Following the formula above if you want to get degrees Celsius you have to subtract 32 from degrees Fahrenheit multiply the result by 5 and divide it by 9.
So in case with 9 we have: ((9 - 32) × 5) ÷ 9 = (-23 × 5) ÷ 9 = -115 ÷ 9 = -12.777777777778 °C