According to Conversion formula above if you want to convert degrees Celsius to degrees Fahrenheit you have to multiply degrees Celsius by 9, divide the result by 5 and add 32 to it.
So you have: ((-17.5 × 9) ÷ 5) + 32 = (-157.5 ÷ 5) + 32 = -31.5 + 32 = 0.5 °F