This formula is actually incorrect, you should multiply C by 9/5, and then add 32.
So, the formula is actually
(C * 9/5) + 32 = F
Also, the F=C+32 formula is ver inaccurate, as it would give us -8 degrees Fahrenheit, which is nothing close to the -40 degrees Fahrenheit it is actually equivalent to.
If you were to go by the formula you wrote, we would receive -8 degrees Fahrenheit. However, it is actually -40 degrees Fahrenheit.