Answer:
(11.4 - 22.8) / (26 - 7)
-11.4 / 19 this is the rate at which it burns - the slope
h = (-11.4/19)t + b
22.8 = (-11.4/19)7 + b
22.8 = (-79.8/19) + b
22.8 = -4.2 + b
27 = b
h = (-11.4/19)t + 27
Substitute 15 for t to determine the height of the candle.