The question says that the temprature at 6 PM , was -5° F, and at midnight the temperature dropped 12 degrees.
Now here dropping of temperature means that the temperature goes down , or we can say that goes more negative.
Now at 6 PM temperature was -5, and at midnight it dropped by 12 degrees
So At midnight the temperature was ,....
we subtract 12 degrees from -5 degrees
-5-12 = -17 degrees
So answer is -17°F