Answer: The .30 per mile is the independent, because it is manipulated, and changed throughout. DRY MIX : Dependent, Responding, Y axis. Manipulated, Independent, X axis.
When the slope is zero, the line is horizontal. Its equation will be of the form
... y = constant
The y-value of your point is specified as -9, so the equation of the line through that point is
... y = -9
