Here you go! Hope this helps!!!
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.
Look up online "equation calculator" and it should be the first one to appear, "Symbolab" supper easy way to help you with problems!
Can you give me an example? Thank you