Suppose that the height (in centimeters) of a candle is a linear function of
the amount of time (in hours) it has been burning.
Let x be the amount of time in hours
Let y be the heoght of a candle in centimeters
The two points are then as (9,24.5) and (23,17.5).
Now plug in x=21, we get
Thus the height of the candle after 21 hours is 18.5 centimeters.