-4.2 degrees. What you would do is from 8-12 (midnight) is 4 hours. So it dropped -16.8 degrees in those 4 hours so -16.8 / 4 is -4.2 degrees. So it dropped -4.2 degrees every hour but it was also -4.2 degrees at 9:00 p.m.
What is the question supposed to be?
This equation can have a number of solutions. Typically when solving a two-variable equation, you are looking for the slope intercept form of the line. In this case that is:
y = 3x - 1
