In this case the temperature depends on the hour. So temperature is your X and hour is your Y variable. When h=0, x=0. So the 21*C is you x=0 or y-int point.
If we are trying to match the equation for a line of y=mx+b, 21*C=b.
Our rate of change is -4*C/h, which is our m variable.
We are trying to find miles/hour, which shows that we are going to be dividing the total number of miles by the total number of hours. Thus, in this case, the miles/hour rate will be:
