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: ((-28.8 × 9) ÷ 5) + 32 = (-259.2 ÷ 5) + 32 = -51.84 + 32 = -19.84 °F
