So when the time = 8 hours, the 
candle's height = 13.5 inches

So when the time = 9 hours, the candle's height 
will be 13 inches.

Let x = the number of hours the candle has been 
burning

Let y = the height of the candle when it has been 
burning for x hours.

So when x = 8, y = 13.5 and
when x = 9, y = 13

We want to find the equation of the line that passes
through the two points (8,13.5) and (9,13)
 
First we use the slope formula


where (x1,y1) = (8,13.5)
and where (x2,y2) = (9,13)



Point-slope formula:
y - y1 = m(x - x1)
where (x1,y1) = (8,13.5)

y - 13.5 = -0.5(x - 8)

y - 13.5 = -0.5x + 4

       y = -0.5x + 17.5

Substitute 2.5 for x

       y = -0.5(2.5) + 17.5
       y = 16.25

So the candle was 16.25 inches tall when it had been 
burning for only 2.5 hours.

Edwin