The temperature, t, in burrtown starts at 21°f at midnight, when h = 0. for the next few hours, the temperature drops 4 degrees every hour.
which equation represents the temperature, t, at hour h?
a. t = 4h + 21
b. t = –21h + 4
c. t = 21h + 4
d. t = –4h + 21

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.

So y=(-4)x+(21)

OR in this case, t=-4h+21.

Answer D.